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Mining Topic: Illumination

What is the health and safety problem?

Machine mounted luminaries

Machine mounted luminaries

A significant challenge in an underground mine is providing adequate lighting for mine workers to work safely. An underground mine is the most difficult environment to illuminate according to the Illuminating Engineering Society of North America (IESNA). A dynamic environment, an underground mine includes dust, confined spaces, and surfaces that reflect light poorly and offer low visual contrasts. Lighting is critical to mine workers since they depend heavily on visual cues to see fall of ground, pinning and striking hazards, and slipping and tripping hazards. Consequently, illumination greatly affects mine workers’ ability to perform their jobs safely.

Age is a significant factor affecting one’s visual abilities. The physiology of the human eye is such that visual performance degrades as a person ages. These physiological changes include reduced pupil size and cloudier lenses, which results in less light reaching the retina. As an example, there is 40% less light reaching the retina of a 45-yr-old person compared to a 24-yr-old person. Also, there is a reduction in the number of rod photoreceptors that play a dominant role in vision as light levels decrease. Thus, it can become more difficult for older mine workers to see various hazards, and they can be more sensitive to glare which can cause eye discomfort or reduce their ability to see various hazards.

What is the extent of the problem?

Many of the higher frequency risks in mining are related to the challenge of inadequate lighting. These include slip, trip, and fall (STF) hazards which can be more difficult to detect in low light. Mine Safety and Health Administration (MSHA) accident data for 2008-2012 indicates that Slips, Trips, and Falls (STFs) are the second leading accident class (19.1%, n=1,820) of nonfatal lost-time injuries at underground mining work locations. For this period, STFs resulted in 108,587 total days lost from work. Inadequate light can also prevent a mine worker from seeing an approaching machinery hazard; 38 mine workers were fatally pinned or crushed by continuous mining machines between January 1983 and February 2014.

Age is also an important factor that needs to be considered in illuminating the underground mine environment, given that the average age of the mining workforce is 43 years. As the mining workforce ages, the need for effective underground lighting becomes even more pressing.

How is OMSHR addressing this problem?

The Office of Mine Safety and Health Research (OMSHR) is conducting mine illumination research to improve mine worker safety by improving a mine worker’s ability to see mine hazards. Thus far, 16 papers have been published, covering diverse topics such as cap lamps, machine-mounted lighting, glare, lighting maintenance, postural control and stability, and light-emitting diode (LED) technology issues. OMSHR researchers have also developed an LED cap lamp, LED area lighting, and a machine Visual Warning System to address struck-by and pinning accidents.

LED cap lamps were developed in two research phases. The phase I cap lamp focused on enhancing the color of light. The phase II cap lamp extended the gains from the phase I cap lamp by changing the lighting distribution such that floor and moving machinery hazards received more light to make them more visible. The phase II LED cap lamp is also approved by the Mine Safety and Health Administration (MSHA).

One method to address the issue of less light at the retina of an older person is to increase illumination; however, this approach also increases glare and decreases the availability of battery power. OMSHR’s approach to address age-related issues is to manipulate the visible light color spectrum of a cap lamp. At low-light (mesopic) ambient conditions, such as those found in underground mining, an increased short-wavelength spectral content can improve visual performance because the eye is more sensitive to that wavelength of visible light. Therefore, OMSHR developed an LED cap lamp that enhanced these short wavelengths of light.

What are the significant findings?

A comparative study was conducted using the phase I prototype LED cap lamp with short wavelength enhancements, a commercial LED, and an incandescent cap lamp. The results indicated significant improvements for the oldest age group (> 50 yrs old) of test participants; 23.7% faster floor hazard detection; 15% faster peripheral motion detection (essential for detecting moving machinery hazards); and 53.8% reduction in disability glare. With the enhanced LED cap lamp, there was also 65% less power usage compared to the incandescent cap lamp.

In testing, the phase II LED cap lamp enabled 94% faster trip hazard detection and 79% faster peripheral motion detection. With the phase II LED cap lamp, there was no increase in glare and up to 50% less power usage compared to commercially available LED cap lamps.

Human subject testing of the Visual Warning System, as mounted on a continuous mining machine, improved the ability to detect machine movement hazards by 71%. Significantly, this improvement translates up to 1.5 feet of machine movement.

What are the next steps?

The latest research addresses the specific needs for metal/nonmetal mining, where the visual environment and visual tasks are different compared to coal mining; hence, an LED cap lamp for metal/nonmetal mine workers is being developed. Other areas of current research include illumination for rescue chamber deployment and inspection, and research to determine if lighting could be used to improve miner escape and rescue in smoke.

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