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Talking Photometry: Photobiological Safety Testing - Don't Get Burnt

The photobiological safety of artificial optical radiation is a hot topic. Learn about BS EN 62471 and the Artificial Optical Radiation Hazard Directive in our Guide to Photobiological Safety - and avoid getting burnt.

 

Safety   The European Artificial Optical Radiation Hazard Directive (2006/25/EC) imposes a statutory responsibility upon employers (via the UK's Heath and Safety at Work Act) to ensure that employees are not subjected to hazardous exposure levels from lamps and other forms of artificial lighting. This in turn means that manufacturers and suppliers of lighting products are being asked more and more often to provide expopsure limit information in accordance with IEC 62471:2006 and BS EN 62471:2008, "Photobiological Safety of Lamps and Lamp Systems". To satisfy EN 62471, measurements of spectral irradiance and spectral radiance must be performed over the wavelenth range produced by the light source under test; the standard applies to light sources in a very wide wavelength range, from 200-3000nm.

EN 62471 considers the photobiological effects on both skin and our eyes of artificial optical radiation (but not sunlight) and provides a definition of safe exposure limits and a framework for classification. Much like solar UVB radiation (280-315nm) is known to cause "sun burn" (erythema), optical radiation in the 200-3000nm range presents different hazards, depending upon the spectral distribution and actual light level present.

For skin and corneal exposure (and for the light from small blue light sources), we consider the spectral irradiance of the optical radiation, in units of Watts per sq. meter per nm. The absolute spectral irradiance is first measured, and then integrated between defined spectral limits and (in most cases), weighted with a hazard function and then a modified irradiance value calculated (W/m2). This is then compared with the exposure limit defined in the standard and the permissible exposure time to the hazard determined.

For retinal exposure, we are concerned not with spectral irradiance but with spectral radiance, the radiant flux per unit wavelength that reaches the eye per unit solid angle and per unit area, in units of Watts per sq. meter per steradian per nm. The absolute spectral radiance is first measured, and then integrated between defined spectral limits and (in most cases), weighted with a hazard function and then a modified radiance value calculated (W/sr.m2). This is then compared with the exposure limit defined in the standard and the permissible exposure time to the hazard determined.

EN 62471 invokes four spectral weighting functions, three of which are photobiological hazard functions, the fourth being the standard CIE photopic observer function. The photobiological hazard functions are: actinic UV; blue light hazard; and retinal thermal hazard.

Sources of optical radiation are classified by EN 62471 into four hazard category groups, which are based upon the permissible exposure time before the hazardous dose is exceeded:

 

   ► Excempt - No photobiological hazard
   ► Risk Group 1 - No photobiological hazard under normal behavioural limitations
   ► Risk Group 2 - Does not pose a hazard due to aversion response to bright light or thermal discomfort
   ► Risk Group 3 - Hazardous even for momentary exposure

When evaluating a source of optical radiation in accordance with EN 62471, irradiance must be measured at an appropriate distance. Remember that irradiance varies with the inverse square of the distance between the light source and the receiver. For what are termed "GLS" sources (General Lighting Service), the irradiance must be measured at the distance which the light source produces 500 lux. This generally means that the evaluation distance can be several meters. For non-GLS sources, the measurement distance is defined at 200mm.

The equipment necessary to perform photobiological safety testing is relatively expensive and a high degree of expertise is necessary to perform the measurements correctly and calculate the appropriate risk groups for the light source. The good news is that Photometric Testing has both the equipment and expertise, so why not let us take the heat? That way, you don't get your fingers burnt.