NIOSH Mining: Visibility

Step 2 - Identify, Rate and Locate Visual Features

Multiple sources of information are available to satisfy any given information requirement when operating mobile equipment underground. Sometimes redundant sources exist. In other cases, several individual sources must be combined to satisfy the requirement. Not all sources of information are visual; some are auditory and others are tactual. For assessing line of sight, the concern is with visual features that serve as sources of information.

Identifying Visual Features

Specifying information requirements is easy given a thorough knowledge of the individual tasks obtained through a task analysis. Unfortunately, specifying the visual features that provide the information can be very difficult. The principal reason is that there are multiple sources of information. Usually, operators are not aware of how they get the information, only that they become aware of it. To develop visual requirements, specific visual features must be identified that serve as the sources of the information required to operate the equipment safely and efficiently. (The table below lists the principal visual features that serve as sources for the information requirements of shuttle car operators.) As with specifying information requirements, the development of this list of visual features is based on an understanding of the tasks that the operators need to perform.

**Visual Features Associated with the Tramming, Loading and Dumping Information Requirements for shuttle car operators**
Tramming - Position of the machine relative to ribs, roof, and corners
1	Rib same side
2	Rib opposite side
3	Crosscut corner
4	Roof
5	Centerline roof marks
6	Leading corner of the machine, same side
7	Leading corner of the machine, opposite side
Tramming - Location of obstacles and hazards
1	Roof obstructions: (roof bolts, cross beams and straps, hanging cables and pipes, flexible and rigid ventilation tubing, low areas and kettlebottoms, overhead conveyor belts, spalling)
2	Roadway obstructions (tools and supplies, other mobile equipment, personnel, standing water, ditches and holes, rails, brattice, trailing cables, rubble)
3	Rib obstructions (brattice, rubble along rib, irregularities in rib, rib roll, bulges or spalling, ventilation tubing, cables)
Tramming - Speed of movement
1	Rib same side
2	Roof
3	Roadway ahead of the machine
Tramming - Location of the machine within the mine
1	Signs
2	General surroundings
3	Roadway ahead of the machine
Loading - Relative rate of material off the tail-boom
1	Material coming off the boom
Loading - Position of the vehicle relative to the continuous miner
1	Tail boom
2	Rear corner of continuous miner
Loading - Location of obstacles and hazards
1	Roof obstructions (roof bolts, cross beams and straps, ventilation tubing, low areas and kettlebottoms, spalling
2	Roadway obstructions (tools and supplies, personnel, ditches and holes, trailing cables)
3	Rib obstructions (brattice, irregularities in the rib, rib roll, bulges or spalling, ventilation tubing, cables)
Loading - Position and height of the load on the vehicle
1	Top of load
2	Slope of load
Dumping - Position of the vehicle relative to the dump site
1	Front end of dump site
2	Same side leading corner of vehicle
3	Dump site activation pull cord or lever
4	Ramp
Dumping - Position of the load on the vehicle
1	Top of load
2	Back slope of load
Dumping - Location of obstacles and hazards
1	Edge of loading ramp
2	Personnel
3	Tools, supplies
4	Low roof
Dumping - Status of the dump site
1	Center of the dump site
2	Status lights or signal
3	Tender's cap lamp

Rating Visual Features

The importance of visual features depends partly on their likelihood of occurrence, which varies from mine to mine. One example is roof hazards. In mines with an undulating roof and low hanging roof bolts, roof hazards can be important sources of information to avoid mishaps. On the other hand, when you have a good, flat, high roof, seeing it is less important.

Redundant visual sources are usually associated with a given information requirement. An example is the same-side leading corner of a machine and the opposite-side leading corner. Being able to see one corner generally gives the operator enough information to find the other. So how do you rate each corner? If you rate each without regard to the other, they might both get high ratings, implying that the operator must see both. If you rate each assuming the other can be seen, both might get low ratings, implying that neither must be seen.

Possible criteria that can be used to rate individual visual features include:

How good is the cue for supplying the needed information?
How often will it occur in a mine?
How likely is it to be visible given the current configurations of underground equipment?
If operators do not see the visual feature, what effect will it have on accidents and productivity?

Given the complexity of rating visual features, we need a simplifying approach. The suggested method first rates the importance of the information requirements and classifies them into importance categories. The importance categories comprise two three-point scales dealing with safety and productivity (see the table below).

**Importance category scales**
Safety - If the information is not received:
2 - Major injury or property damage is likely to occur
1 - Minor injury or damage may result
0 - Unlikely that injury or property damage will result
Productivity - If the information is not received:
2 - Quantity and quality of production will be greatly affected
1 - Quantity and quality of production will be somewhat affected
0 - Unlikely that production will be affected

Each of the information requirements should be rated by persons familiar with the operation of the equipment, preferably those who participated in the field observations and interviews that resulted in the development of the information requirements. A group decision-making technique (such as Delphi) can be used to resolve discrepancies in the ratings. The ratings of the two scales listed in the table above are then averaged and the averages added for a total importance rating. The table below lists the information requirements for shuttle cars showing the average scale rating and total importance rating for each.

**Importance Ratings for each Information Requirement for Shuttle Cars**
Information Requirements for Shuttle Cars		Mean Ratings			Priority
Information Requirements for Shuttle Cars		Safety	Productivity	Total	Priority
Tramming
1	Position of the machine relative to ribs, roof, and corners	1.75	2.00	3.75	1
2	Location of obstacles and hazards	2.00	2.00	4.00	1
3	Speed of movement	2.00	1.00	3.00	2
4	Location of the machine within the mine	0.00	0.75	0.75	4
Loading
1	Relative rate of material off the tail-boom	0.25	0.75	1.00	3
2	Position of the vehicle relative to the continuous miner	0.00	2.00	2.00	3
3	Location of obstacles and hazards	1.75	1.00	2.75	2
4	Position and height of the load on the vehicle	1.00	2.00	3.00	2
Dumping
1	Position of the vehicle relative to the dump site	0.00	1.75	1.75	3
2	Position of the load on the vehicle	0.25	0.00	0.25	4
3	Location of obstacles and hazards	1.75	2.00	3.75	1
4	Status of the dump site	0.00	2.00	2.00	3

The personnel who took part in the study of 12 underground coal mines mentioned previously generated these ratings. The table below is the frequency distribution of the importance ratings for shuttle cars. The ratings were divided into four priority categories for specifying visual requirements. The visual features take on the importance rating of the information requirement of which they are a part. Where two or more visual features serve as redundant sources of information, those features ahead and/or on the same side of the machine as the operator are given preference over features behind and/or on the opposite side of the machine. This preference takes into account the general layouts of equipment in which designing for same-side viewing will be easier than for opposite-side viewing.

**Frequency distribution of importance ratings and Priority Categories for shuttle cars**
Total Rating	Frequency	Priority	Definition
4.00	/	1	No head or torso flexion should be required to obtain information
3.75	/ /
3.50
3.25		2	Reasonable head flexion permitted to obtain information
3.00	/ /
2.75	/
2.50
2.25		3	Reasonable head and torso flexion permitted obtain information
2.00	/ /
1.75	/
1.50
1.25
1.00	/
0.75	/	4	Major body repositioning permitted to obtain information
0.50
0.25	/
0.00

Locating Visual Features

The location of a visual feature can be specified by its position in three planes: fore-aft, lateral (side-to-side), and vertical (up-down). To do this requires that reference points in each plane be identified. Ideally, the reference points, and therefore the location of the visual feature, should be determined so that they can be generalized across specific equipment types. For example, a continuous miner operator needs to see the cutting head of the machine. If the fore-aft location were specified with reference to the operator's head position, the location of the cutting head would vary depending on the length of the machine and the location of the operator's compartment. However, if the fore-aft position is specified with reference to a machine point, in this case the front edge, it does not matter what the length of the machine is or where the operator is positioned.

The philosophy underlying the specification of visual feature locations is that the location should represent the last point at which the information, if received, can be used by the operator. For example, consider a road obstruction, such as a pile of timber, and its location in the fore-aft plane. How far in front of the vehicle must the operator be able to see the obstruction? To see it 200 feet ahead would be nice, but it is not really necessary. The location where it must be seen is a necessary stopping distance ahead of the machine. This necessary stopping distance depends on the speed of the machine, the reaction time of the operator, and the inherent stopping capability of the machine.

Appendix A contains the complete list of visual features for shuttle cars, organized by information requirement, and their location in the three planes (fore-aft, side-to-side and up-down). There is also a list of abbreviations used to specify the locations.

Roadway Obstacles

Some explanation of roadway obstacles is necessary. (Reference the figure below for an illustration of the roadway hazard zone for moving equipment.) It is important that the operator see obstacles in the shaded zone of the illustration. Obstacles not in the shaded zone are either no threat, or even if the operator saw them, little can be done to avoid them. The shaded areas outside the widest machine points represent locations where if obstacles are seen, the operator will not attempt a change in direction (because there is no chance of stopping the machine in time to avoid hitting them). Note that as the machine moves or turns, these shaded areas also move and turn with the machine.

Rather than specify an area, specific points in the area that define the perimeter and midline are specified. These are marked as numbered circles in the illustration; the rationale is that if these points can be seen, effectively the area can be seen. This approach will later simplify the specification of visibility requirements.

Roadway hazard zone for moving equipment

Step 3 - Specifying Visibility Requirements

There are two generic approaches for specifying visibility requirements. The first is to specify "visual windows" of given size and location. The second approach is to specify specific points in space that must be visible. Note that neither approach deals directly with methods for assessing whether a particular piece of equipment meets the requirements.

Visual Windows

Illustration of visual windows Visual windows are areas of unobstructed vision. Visual windows must be specified using a visual angle, rather than absolute size (unless the distance from the operator to the window is also specified). For example, specifying a window 20 in by 8 in (50.8 cm by 20.32 cm) in the forward direction is meaningless. Such a window, 10 in (25.4 cm) from the operator's face will provide much more visual access than if the same window were placed 10 ft (304.80 cm) in front of the operator (see the illustration). If the distance is specified, the visual angle can be computed. The farther away from the observer the window is placed, the larger it must be to maintain the same visual angle. Appendix B has more discussion of Visual Windows.

Visual Attention Locations (VAL's)

The second approach to specifying visual requirements eliminates the need to translate visual features into visual angles. The approach specifies the requirements using specific locations, or visual attention locations (VAL's), which must be visible from the operator's position. The key to the approach is to specify the locations with reference to machine points. In this way, the requirement will apply to all configurations of the equipment class. For example, operators may be required to see an object on the ground a necessary stopping distance (NSD) ahead of the machine. This point can be located in space as shown in the table below.

**Location in space to see an object on the ground a Necessary Stopping Distance ahead of the machine**
Fore-Aft	Front edge of machine + Necessary Stopping Distance
Lateral	Machine center line
Vertical	Floor

The NSD referenced in the table depends, in part, on the speed and braking capacity of the particular model of the machine. Once the NSD is determined (or estimated), the above requirement does not change if the length of the equipment changes, the operator's posture or position changes, or if the width or height of the machine changes. Thus, the requirements are generalizable to all equipment within a given class, i.e., all continuous miners, all shuttle cars, etc. This ability to generalize is one of the most compelling reasons for using this method to specify visual requirements. Appendix C has a discussion of the philosophy of specifying the NSD.)

When you use this method to specify visual requirements, you want to maximize the number of visual features accounted for by using a minimum number of VAL's. To accomplish this, you can combine all of the visual features for a particular class of equipment into a composite graph. This composite can be used to identify points of commonality among the visual features. You should also consider redundancy among visual features associated with the same information requirement.

The operator has three alternatives for movement to be able to see a VAL:

Without head or torso flexion
With head flexion, but no torso flexion
With head and torso flexion.

The alternative selected depends on the importance priority assigned to the visual features at the specific location. The highest priority prevails where several features are found at the same place. For example, if at one location there are four visual features with priority ratings 1, 2, 2, and 3, the overall priority rating of that location would be 1.

Visual Attention Locations for Operation of Shuttle Cars

Appendix D has the coordinates (fore-aft, lateral, and vertical) for each of 54 recommended VAL's associated with operation of shuttle cars. For shuttle cars, these 54 requirements apply to both directions of travel.

A total of 86 visual features were identified in shuttle car operations. The 54 recommended VAL's account for 80 (93%) of all of the visual features identified. A more detailed breakdown by visual feature importance priority is shown in the table below. Considering only the visual features with priorities 1, 2, and 3, the 54 requirements account for 95% of all of the visual features.

**Visual Features accounted for by shuttle car Visual Attention Locations**
Visual Feature Priority	Total Number	Accounted for by Requirements	% Accounted for
1	52	51	98
2	15	15	100
3	13	10	77
4	6	4	67

Step 4 - Assessing Fields of Visibility

Steps 1, 2, and 3 resulted in a list of VAL's that contain visual features important for the safe and efficient operation of the equipment. They describe what needs to be seen. This section discusses a methodology for determining whether a particular equipment and operator compartment configuration permits the operator to see those VAL's.

Line-of-Sight Techniques

There have been numerous approaches suggested in the literature for assessing fields of visibility of mobile equipment. (2, 3) These approaches differ in terms of complexity, type of information generated, and utility for the intended application. (Appendix E has a summary of two such techniques - panorama photograph and shadow graph.) Our discussion will focus on the Line-of-Sight techniques.

Line-of-Sight techniques are predicated on the knowledge of what the operator needs to see to operate the equipment. Thus, it is particularly well suited to the present application. There are two variations on the technique. The first is an inside-out technique in which a camera (or observer) is positioned in the operator's compartment. A target is positioned at the VAL and a picture is taken from the cab, or the observer in the cab indicates if he or she can see the target. The main disadvantage of this inside-out procedure is that separate sets of photographs must be made for the 5th- and 95th-percentile operators. Other than that, the procedure is simple to do, requires a minimum of equipment, produces a permanent record of the obstructions, and requires no data reduction - either the target can or cannot be seen.

The second variation of the line of sight technique is the outside-in method. Here a manikin like device is positioned in the operator compartment. The device is designed to show both the 5th- and 95th-percentile eye positions and can be configured to show eye positions associated with neck and torso flexion. The camera is then positioned at each VAL and a picture is taken looking back to the cab. If the "eyes" of the device can be seen, then an operator could see the VAL. The setup is simple, the photographs are readily interpretable, machine obstructions are easily identified, and the consequences of changing the machine's configuration are readily predictable. The added advantage of this system is that a single set of photographs is all that needed is to evaluate visibility with a 5th- or 95th-percentile operator, with or without neck and torso flexion. Also, this technique is readily adaptable to computerized methods. It is for these reasons that the outside-in line of sight method is the suggested technique for visibility assessments.

HERMI and the Outside-In Technique

HERMI The outside-in line of sight procedure is centered around a human eye reference measurement instrument (HERMI) and specific VAL's. HERMI is a device that represents the eye positions of the 5th-percentile female and 95th-percentile male performing reasonable neck and trunk flexion. The HERMI illustrated here has an eye arc that represents the eye positions for the 95th-percentile male.

HERMI is placed in the operator's cab, simulating the position of the operator. At each VAL, the evaluator takes a picture of HERMI in the operator's cab. Examination of the photograph allows direct determination of whether the 5th- and/or 95th-percentile operator could see that location using a reasonable amount of the neck and/or trunk flexion. An alternative to photography is to position a sight tube at the VAL and have an observer report what parts of HERMI are visible. The disadvantage of this, of course, is that no permanent photographic record is made.

Appendix F describes the procedure for assessing visibility using a HERMI type instrument. At least two people are required to complete the procedure efficiently.

How Good is Acceptable?

Based on the limited sample of shuttle cars, continuous miners, and scoops evaluated to develop this assessment technique, seeing some VAL's for underground mobile equipment will be virtually impossible. The critical question is whether all the VAL's must be seen in order for a machine to have an adequate field of view. The answer, undoubtedly, is "no." This is the logical consequence of the fact that visual features at one VAL can be redundant sources of information to visual features at another VAL.

You can approach the question of how good is acceptable visibility by envisioning a scale of "goodness" ranging from high to low. The question then transforms itself into assigning a cutoff point along the scale that separates acceptable from unacceptable levels of goodness. The first step toward establishing this cutoff is to attempt to quantify the evaluation of visibility so that specific equipment can be placed on the scale of goodness. To do this, we can list some factors that should influence the level of goodness associated with a particular machine:

The more VAL's that can be seen, the better it is (i.e., the higher the level of goodness).
The less body movement (i.e., neck and torso flexion) required to see the VAL, the better the visibility from the equipment.
The greater the percentage of operators that can see a VAL, the better the visibility from the machine. If the 95th-percentile operator could see the location but the 5th-percentile operator could not, this would be less acceptable than if both the 95th- and 5th-percentile operators could see the location.
The higher the priority (i.e., 1, 2, 3, 4) of the VAL that can be seen, the better is the visibility from the machine. If an operator can see a priority 1 VAL, that should get more "weight" than if a priority 3 VAL can be seen.

Based on these factors and the notion of a scale of goodness, developing a system by which a visibility score from a particular machine can be computed is possible using the HERMI technique. Such a scoring system is described below. The system can be used to explore alternative equipment designs, such as the impact of canopy height on visibility for operation of underground equipment.

The procedure assigns points to each VAL based on body movement required to see the location and the priority of the location. These points are determined separately for the 95th-percentile male operator and the 5th-percentile female operator and added. The total is then weighted by the priority of the visual attention area. This composite is then summed over all visual attention areas. This can be expressed by the following formula:

V = Visibility score
Pi = Priority weight for Visual Attention Location i
Ui = 95th-percentile (upper eye arc) points for Visual Attention Location i
Li = 5th-percentile (lower eye arc) points for Visual Attention Location i

The upper and lower eye arc points (i.e., Ui and Li), corresponding to the 95th-percentile male and 5th-percentile female operator, are determined using the table below.

**Points to be assigned to Upper and Lower Eye Arcs of HERMI for determining visibility index. The logic behind this scoring technique is explained at the bottom of the table.**
Body Movement Required	Visual Attention Location Priority Level
Body Movement Required	1	2	3
None	3	3	3
Neck Flexion only	2	3	3
Neck and Torso Flexion	1	2	3
Not Visible	0	0	0
Priority 1 VAL's should require no neck or torso flexion in order to see them. Thus, if the location can be seen without flexion, three points are assigned. If neck flexion is required, only two points are given; if neck and trunk flexion are required one point is given. No points are given if the location is not visible at all.
Priority 2 locations could require neck flexion to be seen. Thus, if no flexion or just neck flexion are required, three points are assigned. If torso flexion is required, two points are assigned, and if the location is not visible, no points are assigned.
Priority 3 locations could require neck and torso flexion to be seen, because they are of less importance to the safe and efficient operation of the machine. Thus, three points are assigned if the location is visible, and no points, if it is not.

This scheme is applied to both the upper and lower eye arcs of HERMI. In each case, the highest number of points attained are assigned. For example, if at a priority 1 visual attention area, the photograph shows that the entire upper eye arc is visible, but only a part of the torso-flexion portion of the lower eye arc is visible, the following points would be assigned:

Thus, a priority 1 has a priority weight of three, a priority 2 has a priority weight of two, and a priority 3 has a priority weight of one. (There are no visual attention areas with priority values of 4.)

This scoring technique is only one of several that could be employed. For example, a machine might be considered acceptable if, and only if, all priority 1 VAL's are visible. Such a system, however, must take into account redundancy between visual features at different VAL's. Further, such a system would not be sensitive to design changes that reduce or enhance visibility above the acceptable level. The particular scoring technique chosen depends on the purpose for which it is intended to serve. For evaluating design changes, the visibility index scoring system described above would be useful. For specifying minimum requirements, perhaps the if-and-only-if priority 1 system would be more appropriate.

The visibility index system described above has one drawback. It is an additive system (i.e., totaling up points across Visual Attention Locations) and as such, it is a compensatory system. It is possible that a machine design could obstruct vision to a few Priority 1 VAL's and yet obtain a relatively "high" visibility index by affording visibility to all Priority 2 and 3 locations. In like manner, a machine could afford visibility to all Priority 1 VAL's and yet score relatively "low" because vision is obstructed to all Priority 2 and 3 locations.

Basic Illumination Terminology

Note: An excellent summary document on mine lighting entitled "Underground Coal Mine Lighting Handbook" was published in 1986 by William H. Lewis. Part One (USBM IC 9073) covers the basics of lighting. (4) Part Two (USBM IC 9074) covers the application of lighting in underground coal mining. (5)

Unfortunately, describing the concepts related to the measurement of light is made more difficult due to the two systems of measurements used: the United States system and the International System of Units. Because MSHA regulations related to lighting underground equipment still use the U.S. system terminology, the following definitions will be related in those terms.

Illumination (also called illuminance) is a measure of the areal density of light reaching an object or surface. The common U.S. unit of measurement is the footcandle (fc). A footcandle is the density of light falling on the inner surface of a sphere of 1 foot radius when a point source of light with an intensity of 1 candela is placed at the sphere's center. The illumination diminishes according to the cosine of the angle between the surface and the direction of incoming light. The illumination also diminishes with distance according to the inverse square law as illustrated below.

Luminance (L) is the amount of light per unit area reflected from or emitted by a surface. For most purposes, this is the important measurement, because a portion of this light is what usually enters the eye. Although this measure is frequently called brightness, strictly speaking, brightness is the resulting subjective sensation and is influenced by contrast, adaptation, and other factors besides the physical energy in the stimulus. Luminance is commonly expressed by a variety of units. The one with the most relevance for underground mine lighting is the footlambert, which is a unit of luminance equal to that of a perfectly diffusing and reflecting surface illuminated by 1 footcandle.

With these definitions, it is possible to define contrast. Contrast is a measure of luminance difference, usually between that of the luminance of the object of interest and the luminance of the background against which the object is seen. It is computed by following formula:

Contrast can vary from 100% (positive) to zero for objects darker than their backgrounds, and from zero to infinity (negative) for targets brighter than their backgrounds.

See the figure below for an illustration of the concept of contrast. As the shading of the block on the right becomes lighter and the contrast between the blocks becomes greater, the visibility of the vertical border between them increases. (Note: You need at least a 256 color display to see this example properly.)

**How contrast effects visibility.**

The human eyes sensitivity to contrast is affected by the level of illumination. If the illustration is viewed in a dimly lit room, the point where it is possible to distinguish the border would be farther down the figure. Stated another way, the less light there is, the higher the contrast necessary between objects to distinguish them.

Other factors that influence the ability to perceive the border include adaptation level, observer age, object size, time, and movement.

U.S. Mine Lighting Standards

The MSHA standards for underground coal mine lighting are only intended to specify minimum requirements for meeting the primary visual needs at the mine face. Current lighting knowledge and the variability of mining circumstances make specification of more extensive standards inappropriate. However, to truly optimize visibility, exceeding these minimum requirements may occasionally be necessary, such as when trying to illuminate VAL's.

The heart of the underground coal mine lighting standard is expressed in Section 75.1719-1(d), Title 30 of the CFR: "The luminous intensity (surface brightness) of surfaces that are in a miner's normal field of vision of areas in working places that are required to be lighted shall be not less than 0.06 footlamberts."

The basis for the illumination regulations is research that shows that the 0.06-fL level is adequate for performance of most mining tasks where fine perception of details is not required. In other words, this illumination level provides enough contrast between the objects normally needed to be seen during mining and the background provided by the mine environment (usually the coal seam). This result was based on a study of visual tasks in mining that took into account such physical components as distance, duration of observation, target identity, target brightness, background type, and the age of miners. The table below summaries some of the recommended luminance levels for tasks associated with shuttle car operation based on this study.

**Recommended luminance levels for tasks associated with shuttle car operation**
Visual Tasks Primarily Associated with the Shuttle Car Operator	Recommended Task Background Luminance Levels in Footlamberts
	50% of Measurement Error	95% of Measurement Error
Locate miner against conveyer from shuttle car seat	0.0165	0.0425
Locate miner against roof from shuttle car seat	0.0165	0.0425
Determine end of loading by viewing coal on the miner conveyor	0.0631	0.1670
Locate miner conveyor from shuttle car seat	0.0107	0.0263
Detect position of shuttle car vs. rib (near side) while driving outby	0.0929	0.1700
Detect position of shuttle car vs. rib while driving inby (far side)	0.0142	0.0298
Detect empty conveyor on shuttle car when unloading	0.1220	0.2260
Detect black cable against rib to locate machine	0.0851	0.1580
Locate yellow cable (electrical) against rib	0.0461	0.0877

Where greater levels of illumination are desirable for close-in work, it was assumed that the miner's cap lamp would act as a supplement to the general lighting. Also, the 0.06-fL general level of luminance is low enough that adaptation problems in going from the illuminated area to darker areas of the mine are not severe.

The requirement is expressed in terms of luminance since only the physical measure of luminance correlates to what the eye sees. Unfortunately, luminance, which is a measurement of reflected light, is more difficult to determine than illumination, which is a measure of incident light. However, since the nominal reflectivity value for coal is 3%, approximately 2 footcandles of illumination is needed to produce the required luminance of 0.06 fL.

The wording of the lighting standard is important. The statement that 0.06 fL applies to "surfaces that are in a miner's normal field of vision" implies that mine lighting should accommodate peripheral vision. Peripheral vision is important in mining for early recognition of conditions that might give pre warning of potential safety hazards in the peripheral field and performance of tasks that require knowledge of the relative spatial relationships among objects separated by significant distance. Lighting all surfaces in a miner's normal field of vision eliminates the "tunnel vision" effect of the narrow cap lamp beam and overcomes shadowing by machine or roof support structures.

The following is an example of the interpretation of this lighting standard by MSHA for underground coal mine shuttle cars (Section 75.1719-1(e)(6), Title 30, Code of Federal Regulations).

**MSHA Illumination Requirements for Shuttle Cars**
1. Lighting fixtures shall be installed on each end of such other self-propelled equipment which will illuminate face or rib coal surfaces within 10 feet of the front and the rear of the machine to a surface brightness of not less than 0.06 footlamberts. 2. The height and width of the area of the coal surface that is required to be illuminated shall be equal to the height and width of the machine on which the light fixtures are installed. When determining the height and width of the coal surface to be illuminated as provided in Section 75.1719-1(e)(6)(b), the maximum height of the equipment (including sideboards, cabs, and canopies) and the maximum width of the equipment (including bumpers, tires, cabs, and canopies) shall be used. The height and width of the coal surface that is required to be illuminated will be the same in both directions of travel.

Unfortunately, the VAL concept was not used as a basis for the lighting standards; therefore, fulfilling the regulatory requirements is possible while having peripheral VAL's receive inadequate levels of illumination to provide 0.06 fL of luminance.

The figure below illustrates the illumination pattern measured around a typical shuttle car. Illumination was measured within the areas bordered by the black lines to the front, rear, and side of the machine. The locations of shuttle car VAL's are indicated by the letters (A' to T' and A to T).

**Illumination levels for inby (A'...T') and outby (A...T) directions of travel**

Determining Illumination at VAL's

As stated previously, if the nominal reflectivity value for coal is 3%, then approximately 2 footcandles of illumination is needed to produce the required luminance (surface brightness) of 0.06 fL. This fact makes possible a simple method to determine the illumination level at VAL's.

The technique uses a photometer and a specially designed light measurement tube (see the cut away sketch of the tube below). The tube is constructed to eliminate all stray light from reaching the photometer. Only the illumination emitted from the luminaire, or stray light reflected off the face of the luminaire, is measured. The use of this measurement tube permits one to measure the illumination level of luminaires in daylight conditions.

Illumination measurement tube

To make a measurement, the measurement tube is positioned at a VAL using a tripod and aimed at a luminaire. The machine light is draped (shaded) with black cloth to reduce the stray light reaching the face of the luminaire. After adjusting the front iris of the tube so that an area slightly larger than the luminaire is visible, the photometer is attached to the rear of the tube and the light reaching the photometer is measured. The luminaire is then turned off and the measurement repeated. This second measure is used to correct the lights-on reading for stray light reflected off the face of the luminaire. The same procedure, lights-on, lights-off, is repeated aiming the light tube at all of the luminaires.

The illumination values taken are conservative, inasmuch as they do not include light reflected from the floor, roof, or ribs. However, because the reflectivity of coal is very low, not much additional light would be added.

An analysis of illumination levels at the VAL's for six shuttle cars revealed that, as a whole, illumination levels meet and often exceed the MSHA shuttle car illumination requirements. Inadequate illumination levels were recorded for VAL's on the side of the machine opposite the operator. It is unlikely that cap lamp illumination would rectify the problem. In contrast, although inadequate levels of illuminance were recorded for some VAL's on the same side of the machine as the operator, it is likely that cap lamp illumination would solve the problem.

Appendix A - Visual Features for Shuttle Cars

The first table describes the abbreviations used to specify the locations of visual features. The second table is the complete list of visual features for shuttle cars, organized by information requirement, and their location in the three planes (fore-aft, side-to-side and up-down).

Some assumptions that were made when locating these visual features for shuttle cars are:

Roadway width:20 ft (609.6 cm).
Shuttle Car width:9.5 ft (289.6 cm).
Distance from the Widest Machine Point on the same side as the operator to the rib: 3 ft (91.4 cm).
Tram Speed: 5 mph (8.0 km), resulting in a Necessary Stopping Distance (NSD) of 20 ft (609.6 cm).


CH	Canopy Height	OCL	Operator Center Line
FE	Front Edge	OEH	Operator Eye Height
HMP	Highest Machine Point	OH	Operator's Head
MBH	Maximum Boom Height	OS	Opposite Side from Operator
MBS	Maximum Boom Swing	RE	Rear End
MCL	Machine Center Line	SH	Seam Height
MCH	Maximum Cutter Height	SS	Same Side as Operator
MMH	Median Machine Height	SST	Safe Stopping Time
NSD	Necessary Stopping Distance	WMP	Widest Machine Point
		WE<	Whichever is Less

Visual Feature	Fore-Aft	Side-to-Side (Lateral)	Up-Down (Height)
Visual Feature	+ forward - rearward	+ away from center - toward center	+ up - down
Tramming - Position of the Machine Relative to Ribs, Roof, and Corners Priority 1
SS Rib	OH + ½ NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
OS Rib	FE + ½ NSD	WMP(OS) + 7 ft (WMP(OS) + 213.4 cm)	OEH
Cross Cut Centers	FE + 20 ft (FE + 609.6 cm)	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
	FE + 20 ft (FE + 609.6 cm)	WMP(OS) + 7 ft (WMP(OS) + 213.4 cm)	OEH
	FE	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
Roof Irregularities	FE + ½ NSD	OCL	SH
Center Line of Roof Marks	FE + ½ NSD	MCL	SH
SS Lead Corner of Machine	FE	WMP(SS)	MMH
OS Lead Corner of Machine	FE	WMP(OS)	MMH
Tramming - Location of Obstructions and Hazards Priority 1
Roadway Obstacles	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	Floor
	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + ½ NSD	Floor
	FE + ½ NSD	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	Floor
	FE + ½ NSD	WMP(SS) + ½ NSD	Floor
	FE + NSD	WMP(SS)	Floor
	FE + NSD	WMP(SS) + ½ NSD	Floor
	FE + NSD	MCL	Floor
	FE + 2 ft (FE + 61.0 cm)	WMP(OS) + 2 ft (WMP(OS) + 61.0 cm)	Floor
	FE + 2 ft (FE + 61.0 cm)	WMP(OS) + ½ NSD	Floor
	FE + ½ NSD	WMP(OS) + 2 ft (WMP(OS) + 61.0 cm)	Floor
	FE + ½ NSD	WMP(OS) + ½ NSD	Floor
	FE + NSD	WMP(OS)	Floor
	FE + NSD	WMP(OS) + ½ NSD	Floor
Roof Obstructions	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	HMP
	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + ½ NSD	HMP
	FE + ½ NSD	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	HMP
	FE + ½ NSD	WMP(SS) + ½ NSD	HMP
	FE + NSD	WMP(SS)	HMP
	FE + NSD	WMP(SS) + ½ NSD	HMP
	FE + NSD	MCL	HMP
	FE + 2 ft (FE + 61.0 cm)	WMP(OS) + 2 ft (WMP(OS) + 61.0 cm)	HMP
	FE + 2 ft (FE + 61.0 cm)	WMP(OS) + ½ NSD	HMP
	FE + ½ NSD	WMP(OS) + 2 ft (WMP(OS) + 61.0 cm)	HMP
	FE + ½ NSD	WMP(OS) + ½ NSD	HMP
	FE + NSD	WMP(OS)	HMP
	FE + NSD	WMP(OS) + ½ NSD	HMP
Rib Obstructions	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	Floor
	FE + ½ NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	Floor
	FE + NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	Floor
	FE + 2 ft (FE + 61.0 cm)	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	HMP
	FE + ½ NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	HMP
	FE + NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	HMP
Tramming - Speed of Movement Priority 2
SS Rib	OH + ½ NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
Roof	FE + ½ NSD	OCL	SH
Roadway	FE + ½ NSD	OCL	Floor
Tramming - Location of The Machine Within The Mine Priority 4
Signs	FE + NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	8 ft (243.8 cm) or SH, WE<
General Surroundings	FE + NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
	FE + NSD	WMP(SS) + 3 ft (WMP(SS) + 91.4 cm)	OEH
	FE + NSD	MCL	OEH
Loading - Relative Rate of Material off the Tail Boom Priority 3
Material off the Tail Boom	FE	MCL	MBH or SH, WE<
Material off the Tail Boom	FE	MCL	MMH
Loading - Position of the Vehicle Relative to the Continuous Miner Priority 3
Tail Boom	FE + ½ NSD	MCL	MBH or SH, WE<
Tail Boom	FE + ½ NSD	MCL	MMH
Rear Corner of CM	FE + ½ NSD	WMP(SS)	MMH
Loading - Location of Obstructions and Hazards Priority 2
Roof Hazards	FE	WMP(SS)	HMP
Roof Hazards	FE	WMP(SS)	HMP
Rib Obstructions	FE	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	MMH
Rib Obstructions	FE	WMP(OS) + 8 ft (WMP(OS) + 243.8 cm)	MMH
Roadway Obstructions	FE	WMP(SS) + 5 ft (WMP(SS) + 152.4 cm)	OEH
Roadway Obstructions	FE	WMP(OS) + 5 ft (WMP(OS) + 152.4 cm)	OEH
Loading - Position and Height of the Load on the Vehicle Priority 2
Top of Load	FE	MCL	HMP
	OH	MCL	HMP
	RE	MCL	HMP
Slope of Pile	RE	MCL	OEH
	OH	MCL	OEH
	FE	MCL	OEH
Dumping - Position of the Vehicle Relative to the Dump Site Priority 3
Front End of Dump	FE + NSD	OCL	MMH
SS Lead Corner of Machine	FE	WMP(SS)	MMH
Dumpsite Activation Lever	OH	OCL + 2 ft (OCL + 61.0 cm)	OEH
Ramp	FE + NSD	WMP(SS)	Floor
Dumping - Position of the Load on the Vehicle Priority 4
Top of Load	FE	MCL	HMP
Back Slope of Load	FE - 4 ft (FE - 121.9 cm)	MCL	OEH
Dumping - Location of Obstacles and Hazards Priority 1
Edge of Loading Ramp	FE + NSD	WMP(SS) + 2 ft (WMP(SS) + 61.0 cm)	Floor
Personnel	FE + NSD	WMP(SS)	OEH
	FE + NSD	WMP(OS)	OEH
	FE + NSD	WMP(SS) + ½ NSD	OEH
	FE + NSD	WMP(OS) + ½ NSD	OEH
	FE + NSD	MCL	OEH
Tools and supplies	FE + NSD	WMP(SS)	Floor
	FE + NSD	WMP(OS)	Floor
	FE + NSD	MCL	Floor
Roof hazards	FE + NSD	MCL	HMP
Dumping - Status of the Dump Site Priority 3
Center of Dump Site	FE + NSD	MCL	MMH
Status Lights	FE + NSD	WMP(SS)	OEH
Tenders Cap Lamp	FE + NSD	WMP(SS) + 5 ft (WMP(SS) + 152.4 cm)	HMP
Tenders Cap Lamp	FE + NSD	WMP(OS) + 5 ft (WMP(OS) + 152.4 cm)	HMP

Appendix C - Philosophy of Specifying the NSD

The philosophy in specifying the NSD is to assume reasonable worst-case situations. To come up with reasonable NSD's, operator reaction time was taken to be 2 seconds. This corresponds well with reaction times reported for automobile braking. For example, researchers have found that 90% of the reaction times measured under actual driving conditions were less than 1.5 seconds. (6) The 2-second value chosen here takes into account the adverse environment encountered underground and larger movement times that might occur due to the less-than-optimum control positions found on many mining machines.

The distance required for the vehicle to stop once the brakes are applied can be determined empirically as outlined in SAE Subcommittee 29, Working Group 8, Recommended Brake Standards for Underground Machines. (7) Maximum tram speed is supposed to be used in the determination. However, since adequate data do not exist on braking distances for specific models of machines, an alternative procedure is used here. The SAE Subcommittee 29 Brake Standard contains minimum performance criteria for brake systems for rubber-tired, self-propelled underground mining machines. These criteria are used as the worst-case braking distances.

The table below presents the Necessary Stopping Distances for various maximum tram speeds using the assumptions discussed above. For very slow-moving equipment (such as continuous miners), the computed necessary stopping distance is less than 10 ft (304.8 cm). Interviews with continuous miner operators revealed a desire to see at least 10 ft (304.8 cm) ahead of the machine during tramming. Therefore, a minimum 10 ft (304.8 cm) necessary stopping distance was assigned. Any vehicle with a maximum tram speed of 2.5 mph (4.02 km) or less is automatically assigned a 10 ft (304.8 cm) NSD.

One point of confirmation for this procedure is that during the shuttle car operator interviews, respondents consistently said that they needed to see at least 20 ft (609.6 cm) ahead of them. Most shuttle cars have a maximum tram speed of approximately 5 mph. From the table below, this translates to a necessary stopping distance of 20 ft (609.6 cm).

Necessary Stopping Distance as a function of maximum tram speed

Maximum Tram Speed (mph)

Reaction Time Distance*
ft (cm)

Braking Distance**
ft (cm)

Necessary Stopping Distance***
ft (cm)

1.0

3.0 (91.4)

0.7 (21.3)

10 (304.8)

2.0

6.0 (182.9)

1.4 (42.7)

10 (304.8)

2.5

7.0 (213.4)

1.8 (54.9)

10 (304.8)

3.0

9.0 (274.3)

2.3 (70.1)

11 (335.3)

4.0

12.0 (365.8)

3.4 (103.6)

15 (457.2)

5.0

15.0 (457.2)

4.7 (143.3)

20 (609.6)

6.0

18.0 (548.6)

6.1 (185.9)

24 (731.5)

7.0

21.0 (640.1)

7.7 (234.7)

29 (883.9)

8.0

23.0 (701.0)

9.5 (289.6)

33 (1005.8)

9.0

26.0 (792.5)

11.4 (347.7)

37 (1127.8)

10.0

29.0 (883.9)

13.5 (411.5)

43 (1310.6)

*	Assumes a 2-second reaction time.
**	From SAE Subcommittee 29, Working Group 8: Brake Standards for Underground Machines. (17)
***	Values rounded off to the nearest whole number. Values less than 10 ft (304.8 cm) are assigned a value of 10 ft (304.8 cm).

NIOSH Mining Safety and Health Topic

Visibility

Introduction

Assessing Line of Sight

Step 1 - Specifying Information Requirements

Step 2 - Identify, Rate and Locate Visual Features

Identifying Visual Features

Rating Visual Features

Locating Visual Features

Roadway Obstacles

Step 3 - Specifying Visibility Requirements

Visual Windows

Visual Attention Locations (VAL's)

Visual Attention Locations for Operation of Shuttle Cars

Step 4 - Assessing Fields of Visibility

Line-of-Sight Techniques

HERMI and the Outside-In Technique

How Good is Acceptable?

llumination Levels at VAL's

Basic Illumination Terminology

U.S. Mine Lighting Standards

Determining Illumination at VAL's

Appendices

Appendix A - Visual Features for Shuttle Cars

Appendix B - Visual Windows

Appendix C - Philosophy of Specifying the NSD

Appendix D - Recommended VALs for Shuttle Cars

NIOSH Mining Safety and Health Topic

Visibility

Introduction Note - This is the printer friendly format of this page. Refresh your browser for the screen version.

Assessing Line of Sight

Step 1 - Specifying Information Requirements

Step 2 - Identify, Rate and Locate Visual Features

Identifying Visual Features

Rating Visual Features

Locating Visual Features

Roadway Obstacles

Step 3 - Specifying Visibility Requirements

Visual Windows

Visual Attention Locations (VAL's)

Visual Attention Locations for Operation of Shuttle Cars

Step 4 - Assessing Fields of Visibility

Line-of-Sight Techniques

HERMI and the Outside-In Technique

How Good is Acceptable?

llumination Levels at VAL's

Basic Illumination Terminology

U.S. Mine Lighting Standards

Determining Illumination at VAL's

Appendices

Appendix A - Visual Features for Shuttle Cars

Appendix B - Visual Windows

Appendix C - Philosophy of Specifying the NSD

Appendix D - Recommended VALs for Shuttle Cars

Introduction