Incoherent Optical Radiation (IOR)

The main reference for the photobiological effects summarized here is the 2013 Guidelines published by the ICNIRP [1], unless referenced otherwise within the text. Two organs are at risk with high-intensity incoherent light sources: the eyes and the skin. Since the eye is more sensitive than the skin to radiation, the thresholds are more restrictive for ocular exposure.

The UV portion of the spectrum is subdivided by the CIE into the UV-A, UV-B, and UV-C bands, as shown here. The infrared (IR) spectrum is also subdivided:

IR-A	(780 nm – 1.4 µm)
IR-B	(1.4 µm – 3 µm)
IR-C	(3 µm – 1 mm)

One of the challenges with incoherent optical radiation (IOR) hazards is the polychromatic nature of the light, so several spectral hazards need to be considered. Another challenge is that these sources are not “point-like” as they are with lasers, so larger area exposures need to be considered: both the skin and the eyes. The following figure illustrates the span of challenges, using the spectrum of a Xe arc lamp. The hazards are annotated on the figure.

Spectrum of a xenon arc lamp with ocular and skin hazards annotated in their respective spectral regions. The IR bands relevant to this source are also given.

Eye

Ocular threshold limits are based on the appearance of lesions in a particular structure of the eye after an acute exposure (within a day of exposure).

Retinal Photochemical hazard (380 nm – 550 nm)

Photochemical damage region, or blue-light hazard region, is the most hazardous part of the spectrum since damage can occur at exposure levels similar to natural light. [1,2]. Over-exposure to this spectral range is attributed as solar blindness or welder’s flash, and can lead to scotoma. Photochemical damage follows the reciprocity rule for exposure time and intensity and is thus separated into two types: Type I results from long-term, low-intensity exposure where damage is due to constant photobleaching; Type II results from intense, short-term exposure

The sensitivity of the retina in this spectral range is given by the blue-light hazard function shown in the figure below.

Retinal thermal hazard (380 nm – 1.4 µm)

Retinal thermal damage occurs when the light exposure level is sufficient to raise the temperature of the retinal tissue, which leads to lesion formation [2]. Very bright sources are needed to cause this type of injury, usually xenon arc-lamps or lasers. In the UV, and IR-B and IR-C, light mainly deposits its energy in the cornea, which limits the retinal hazard to the spectral range indicated in this section.

This hazard is dependent on the retinal image spot size since retinal damage depends on heat conduction away from the exposure location. The sensitivity of the retina to photons in this spectral range is given by the retinal hazard function shown in the figure below.

Cornea thermal hazard (1.4 µm – 1 mm)

Water readily absorbs radiation above 1400 nm [2], so the light in this spectral range generally interacts with the cornea. The major hazard with corneal burns in this spectral range results from a lack of visible stimulus, so the individual is not aware of the presence of the beam until symptoms occur. Chronic exposure of the lens and cornea to IR radiation leads to the development of cataracts.

Calculating the effective exposure

The effective exposure of the eye to a thermal or photochemical hazard using the hazard functions below is simple. Given the spectral radiance of a source Lλ (W/cm2 sr nm), this spectral radiance is multiplied against either hazard function: replace H(λ) with B(λ) for the blue-light hazard or R(λ) for the retinal thermal hazard as shown:

Figure

Ocular hazard functions for the blue-light photochemical damage (labeled and in blue) and the thermal retinal damage (labeled and in red). These functions are applied against the spectral emission of the light source to determine the effective exposure of the eye.

Skin

Exposure limits are based on whether radiation-induced erythema (redness of the skin) occurs within 48 hours of exposure. Usually a very bright, pulsed source is required to cause skin injury.

Thermal Damage (380 nm – 1 mm)

This risk usually occurs where a very high irradiance due to a pulsed source is available. The amount of damage can depend on spot size of the light on the skin.

Photosensitization and Immune Reactions (typically UV)

See Photosensitivity and Biological Effects on the UV webpage.

References

[1] International Commission on Non-Ionizing Radiation Protection, “Guidelines on limits of exposure to incoherent visible and infrared radiation,” Health Phys. vol. 105, pp.74-96, 2013. Available: www.icnirp.org/cms/upload/publications/ICNIRPVisible_Infrared2013.pdf (PDF, 1.2 Mo)

[2] J. Wu, S. Seregard, P.V. Algvere, "Photochemical damage of the retina," Survey of Ophthalmologyvol.51, pp.461-481, (2006). doi: 10.1016/j.survophthal.2006.06.009

Retinal Thermal Hazard Limit (380 nm – 1.4 µm)

The retinal thermal hazard limits in the figure were drawn from the specifications [1]. The effective radiance of a source must not exceed that shown for a given exposure.

Figure

The thermal hazard limit for a source. The exposure time in the equations is in seconds and the angle of the source is in radians.

Retinal Blue-Light Photochemical Hazard Limit (380 nm – 550 nm)

In the 300 nm to 700 nm spectral range, the limit is based on exposure time:

0.25 s < t < 10,000 s (2.8 hours)	100 J/cm²sr (exposure dose)
t > 10,000 s	10 mW/cm²sr

If required, the solid angle used to calculate the irradiance or radiant exposure also depends on time, the following plane angles are valid for the given time of exposure:

t < 100 s	11 mrad
100 s < t < 10,000 s	11 × t^0.5 mrad
t > 10,000 s	110 mrad

Corneal and Lens Thermal Hazard Limit (1.4 µm – 1 mm)

The room temperature corneal/lens limits are irradiance limits based on exposure time.

t < 1000 s	1.8 × t^0.75 W/cm²
t > 1000 s	10 mW/cm²sr

Skin Thermal Hazard Limits (380 nm – 1 mm)

For all wavelengths considered here, the radiant exposure for exposure times less than 10 seconds is 2 ×t^0.25 J/cm². For longer exposure times, natural avoidance will limit the exposure time since pain is induced when the temperature of the skin rises above 45 °C: this temperature is lower than that needed to create a thermal burn.

References

[1] International Commission on Non-Ionizing Radiation Protection, “Guidelines on limits of exposure to incoherent visible and infrared radiation,” Health Phys. vol. 105, pp.74-96, 2013.

A complete discussion on incoherent optical radiation includes a discussion about lumens: a measure of our biological perception of brightness. Since visible, incoherent sources are not monochromatic and used for illuminating objects for personal use, radiometric quantities (W, J) are not used to define their radiant energy. These units do not take our perception of different wavelengths of light into account. For example, 1 W of 532 nm radiation – a green laser pointer – is perceived at 88 % of its total power, whereas 1 W of 633 nm radiation – a red laser pointer – is perceived at 26 % of its total power as shown in following graphic using the data published at CVRL.

Figure

The 2-deg luminous efficiency that shows our optical efficiency to perceive colour. As seen in the figure, we perceive green light much more efficiently than red or blue light.

Instead of radiometric units (W), photometric values are used that correct for our wavelength-dependent eye response. The luminous flux (lumens) provided by an incoherent broadband light source is calculated as follows, (p.52 of [1]):

E_vis is the visual luminous flux in lumens
E_λ is the spectral power distribution (W/nm)
V(λ) is the luminous efficiency function (unitless)
683 is a scaling factor (lumen/W)

The Luminous Efficiency Function V(λ) is the relative psychophysical response of the eye in a wavelength interval as shown above. The function should be used directly as published in the above formula (i.e.: the integral of V(λ)dλ does not equal one) since it scales the relative eye response to a particular wavelength interval.

NOTE: If the light source (ex: LED) emits in the infrared or ultraviolet wavelength ranges, the power delivered by the source should be listed in Watts.

As an aside, our visual response to different wavelengths also indicates how unnecessary it is to have green laser pointers of equivalent power as red laser pointers. Red laser pointers are often about 1 mW, so for an equivalent perceived brightness, a green laser pointer only needs to be 0.25 mW: not the 5 mW that is typically available.

[1] R. Henderson, K. Schulmeister, Laser Safety. Bristol, UK: IoP Publishing, 2004

Evaluation of an Incoherent Source

As an example of an incoherent optical radiation hazard assessment, let's look at a green surface-emitting LED with the following specifications:

Luminous flux	700 lm
Half-maximum emission angle	95 °
Die size	5 mm × 5 mm

Further to this example, the LED emits the following emission spectrum (provided as a normalized curve, the spectral radiance shown below was calculated). We want to convert the normalized curve to a spectral radiance so we can assess the retinal thermal and blue-light hazards associated with this source.

Figure

Emission spectrum of an LED source. Note that the spectral radiance has already been calculated for this curve using the discussion that follows this figure.

To get the actual emission curve, the normalized emission curve and the luminous efficiency function were placed into an spreadsheet program, each with a 0.1 nm wavelength resolution. Taking the total integral (multiplying the columns together and then summing that column), a value of 21,453 was calculated. The expected luminous flux was 700 lm; therefore, a scaling factor of 30.65 (21,453/700) removed the normalization (assuming the instrument used to gather this spectrum had a flat spectral response).

With the normalization removed from the spectrum, integrating the spectrum alone over wavelength (summing the spectrum column) and dividing by the scaling factor gives the optical power emitted into this portion of the visual spectrum of 1.50 W.

Back of the envelope: To check this calculation, note that at the 515 nm peak we have an ocular response of 62.5 % (see Figure). For 700 lm at 515 nm, this luminous flux would correspond to 1.64 W (within 10 % of our calculation).

Our calculated 1.5 W is converted into radiance by dividing by the die area and by half of the solid angle of its half-maximum emission. The area is straight-forward; as for the solid angle, half of the 95 ° planar angle converts to 2.04 steradians (sr).

The total radiance of this source is 2.9 W/cm²sr giving us the spectral radiance (radiance as a function of wavelength) as shown in the figure above to be used for the hazard evaluation.

Hazard evaluation

Now we can assess the retinal thermal hazard and retinal photochemical (blue-light) hazard. Multiplying the spectral radiance curve above by each of the ocular hazard functions (second figure on this page), we get an effective thermal radiance of 2.9 W/cm²sr and an effective blue-light radiance of 180 mW/cm²sr.

Retinal thermal hazard

Looking at the retinal thermal exposure curve, for exposure durations longer than 250 ms, the exposure limit is 2.8/α W/cm²sr where α is the angular size of the LED at the evaluation position, if the source subtends a planar angle between 1.5 mrad and 100 mrad. The worst-case distance for a hazard evaluation is 20 cm, and a 5 mm x 5 mm die at normal incidence would subtend a planar angle of 25 mrad making the effective allowed radiance for this source 100 W/cm²sr. This example would not be a retinal thermal hazard.

Photochemical hazard

The blue-light photochemical exposure limit shows the total allowed dose within a couple of hours to be 100 J/cm²sr. For a source emitting 2.9 W/cm²sr, it would take over 9 minutes to reach this dosage.

Incoherent Optical Radiation (IOR)

IOR Safety Program

Eye

Retinal Photochemical hazard (380 nm – 550 nm)

Retinal thermal hazard (380 nm – 1.4 µm)

Cornea thermal hazard (1.4 µm – 1 mm)

Calculating the effective exposure

Figure

Skin

Thermal Damage (380 nm – 1 mm)

Photosensitization and Immune Reactions (typically UV)

References

Retinal Thermal Hazard Limit (380 nm – 1.4 µm)

Figure

Retinal Blue-Light Photochemical Hazard Limit (380 nm – 550 nm)

Corneal and Lens Thermal Hazard Limit (1.4 µm – 1 mm)

Skin Thermal Hazard Limits (380 nm – 1 mm)

References

Figure

Evaluation of an Incoherent Source

Figure

Hazard evaluation

Retinal thermal hazard

Photochemical hazard