Safety - Understanding IEC 62778 and Using KV,B to Calculate White Light Eye Safety Risk Groups

Eye safety is a complex topic. This article has a focus on White LEDs and the blue light hazard assessment that is almost universally the deciding factor in assigning a risk group for a white light component tested by a certified lab. The K_B,V concept gives the luminaire designer a method to estimate the conditions where a luminaire will be rated below RG2 and therefore not require caution statements.

As background, the table below summarizes the hazards defined in IEC 62471 and the types of LEDs that are generally assigned a risk group based on that particular test. The added protection due to aversion response (about 0.25 seconds) gives visible light sources an added safety buffer so visible RG2 sources require caution but visible RG1 sources do not. This is not the case for nonvisible light where there is no aversion response and, for nonvisible wavelength ranges, exceeding RG0 is cause for caution.

Guidance on risk group exposure precautions for all of the calculations included in IEC 62471

Adapted from Christophe Martinsons, “Photobiological Safety” in Handbook of Advanced Lighting Technology, Springer, 2014.

What is K_B,V?

KB_B,V is a metric introduced in the IEC/TR 62778:2014 standard "Application of IEC 62471 for the assessment of blue light hazard to light sources and luminaires." IEC 62778 is intended to bring “clarification and guidance concerning the assessment of blue light hazard of all lighting products which have the main emission in the visible spectrum (380 nm to 780 nm).”

The improvements in IEC 62778 can be used to estimate LED component risk group classification using knowledge of the CCT and lumen output of a component. Some results can be transferred to a luminaire without further measurements. In the test reports, these are designated as “unlimited” so a RG1 – unlimited component designation under the IEC 62778 report format means that a luminaire using these components can’t have a rating greater than RG1 under any conditions. IEC 62778 blurs the distinction between RG0 and RG1 and the focus is on conditions that create RG2.

How to Calculate K_B,V

The mathematical definition of K_B,V, which has units of W_B/lm is

Where:

W_B is the blue light hazard weighted optical watts of the source.

Ф_λ is the radiometric spectral power distribution of the white light source (W/nm)

λ is the wavelength variable (nm)

B is the blue light hazard weighting function defined in IEC 62478

K_m is the photopic luminous efficacy of radiation for a monochromatic source with a 555 nm wavelength (683 lm/W)

V is the luminous efficiency function (eye response curve).

The magnitude of Ф_λ does not matter in the K_B,V calculation since it divides out. SPDs are often presented in normalized form, and it is unnecessary to scale them before doing the K_B,V calculation.

We have provided an example Excel workbook in the downloads section of this article showing how to do the K_B,V calculation. In simple terms, you multiply the SPD by the blue light hazard function B(λ) to get a summed W_B value for the numerator and multiply the SPD by the eye response curve V(λ) and multiply the summation by K_m (683 lm/W) to get a summed lumen value for the denominator.

This calculator also includes LER (lm/W_optical) since it is a byproduct of intermediate calculations. When known, LER can be used to convert optical watts and lumens for each given SPD. Eye safety calculations use both photometric and radiometric units at different steps in the calculation process.

Screenshot from the provided calculator. K_B,V for the Luminus CLM-14-65-80-36-AC40 is 8.73e-4 blue watts / lumen. We have interpolated the blue light hazard function to 1 nm increments using the table in IEC 62471.

How to Use K_B,V

IEC 62778 provides these additional definitions:

The blue light hazard exposure limits (EL) in IEC/EN 62471 are based on E_B (W_B/m²) and L_B (W_B/sr-m²) values so we can use a calculated K_B,V value to determine the blue light weighted values needed to assign a risk group for a given light source.

The reason that E_B (W_B/m²) and L_B (W_B/sr-m²) values are both used is that eye movements are a factor in the original definitions of exposure limits (EL). The radiance (L) is the basis for exposure limits that consider the spot focused on the retina. For large sources, the radiance is

Where I_source is the radiant intensity of the source in W/sr (from the light source towards the eye). This is independent of distance and can be calculated from the FWHM beam angle of the light source and a few other datasheet values. These calculations are shown in a related article “Optical – How do I calculate lux from an LED datasheet?”

The apparent source area is a strong function of distance and is calculated from the eye towards the light source. Small area sources have a higher W/sr-m² than large sources that have the same I (W/sr) values. IEC 62471 imposes limits on the allowed areas to be used for the apparent source. These limits are expressed as an angle, α, (radians) to simplify the exposure limit (EL) math.

Figure illustrating the parts of the eye and wavelength ranges where L and E values are used. (Adapted from Christophe Martinsons, “Photobiological Safety” in Handbook of Advanced Lighting Technology, Springer, 2014)

For small sources, the true radiance (I divided by actual area) is not used because the high calculated values are considered biologically impossible for times greater than 0.25 seconds. This is because the focus point on the retina is not fixed. The small source image focus is constantly moving around the retina at the back of the eye. For small sources, the Irradiance, E_B, on the target plane is used instead.
This is technically flux on the cornea but in the small source blue light calculation it is used as a surrogate for L.

Recommended method in IEC 62471 for calculating α for square and rectangular sources. The small source limit for α is 0.011 radians. The large source limit is 0.1 radians. In eye safety specifications and calculations α is used as a proxy for the solid angle Ω and the source size.

Visualization of the distinction between L and E. The radiance calculation uses the (W/sr) emanating from the source and the m² of the apparent source looking towards the source.

The most interesting uses of K_B,V are the generalizations that can be performed using CCT as an input variable. An example is shown in the figure below. This is a plot of the “critical lux value” (E_th) versus the CCT of a “small” white light source. This plot is generated by setting the E_B value to 1 W_B/m² which is the blue irradiance threshold where the RG1/RG2 transition occurs. Since K_B,V can be expressed as

setting E_B = 1 gives the result that E_th = 1/K_B,V in lux. Calculating 1/K_B,V for a number of SPDs generates a locus which are shown as orange circles in the figure below.

Reproduction of Figure C.2 in IEC 62778. The blue line is digitized from the original plot. The gray stepped curve is from Table C.2 and incorporates a safety factor of two.  The orange circles are calculated results using Luminus COB components with a variety of CRIs and distances from the black body locus.

A companion article “Optical – How do I calculate lux from an LED datasheet?” shows examples of the lux calculations for lumen and distance inputs. For a numeric example, in the Excel screenshot above, for a 6500 K white LED, the value of KB,V is 8.73e-4 (W_B/m²)/(lm/m²). The reciprocal of 8.73e-4 is 1145.5 lux.

As the CCT moves to warmer color temperatures, the critical lux threshold increases. So, for a 3000 K light source, the luminous irradiance where RG2 occurs is 3000 lux. This means than any light fixture that produces less than 3000 lux at the task plane under consideration is below RG2. While this is intuitively obvious since warm light sources have less blue content per lumen, the analysis that generates the figure below quantifies this relationship in a usable manner.

This is a powerful result for light fixture designers. White light sources at or below RG1 do not require warning labels under IEC 62471-2.

A typical use case for a white light fixture would be targeting 500 lux (or below) on a task plane. According to the figure above, even the highest CCTs do not exceed a 500-lux threshold value so none will exceed RG1 (for blue light) when using a traditional IEC 62471 GLS test.

We have used the small source results in the discussion above because lux (E) is much more intuitive than radiance (L) which is used for a large source. IEC 62778 has a number of special cases due to the physiology of the eye. Eye motion is one of the driving factors behind the differentiation between small and large sources. A similar plot is presented in IEC 62778 using L threshold levels on the y-axis.

IEC 62778 has a number of simplifications. One is that the test distance for IEC 62778 is always 200 mm and the α angle used to differentiate between small and large sources is always 0.011 radians. At 200 mm, this α has a fixed diameter of 2.2 mm.

A light source that overfills a 2.2 mm diameter circle is a large source, and a light source that underfills is a small source. The basis for the risk group limits is based on time exposure limits for different “true radiance” values.

For IEC 62778, the 2.2 mm diameter circle is the only area that needs to be considered. For large sources, this is the area used in the calculation of the true radiance L_B.

LED images showing the 2.2 mm circle used to differentiate between large and small sources in
IEC 62778. For Luminus products, the part number has an area code (SST-10 is 1 mm², SST-90 is 9 mm², etc.) and the SST-20 is the largest product that is a small source. Separated sources / LED arrays are covered in IEC 62778 Annex D. The Global Lighting Association white paper “Optical and Photobiological Safety of LED, CFLs and Other High Efficiency General Lighting Sources” has numerous comparisons of the risk group levels for a variety of white light source technologies. The technical appendix is also worth reading.

The calculations in IEC 72778 use mixed units,

• cd/m² for the initial determination of RG0 unlimited
• W_B/m² or W_B/m²-sr for the determination of RG1 and RG1 – unlimited depending on small or large source size
• Threshold values of lux and distance based on the E_B = 1 W_B/m² threshold condition where the RG1/RG2 boundary occurs.

This decision matrix is shown below. Test labs perform direct measurements, and all of the values are available to the software. For designer estimates, the only calculation needed is the lux value of the source under consideration and Figure C.2 in IEC 62778 (reproduced above for Luminus parts) can be used.

At a most basic level, if a light source is in the field of view and the lux level at the eye position exceeds the critical lux level for that CCT, the source is RG2. Otherwise, it is less than RG2.

Flowchart adapted from Figure 7 in IEC 62778.

For completeness, a table and plot of calculated values are shown below for a variety of Luminus white products.

Tabulated values for a variety of SPDs

P/N	CCT (K)	CIEx	CIEy	Eth (lux) 1/KB,V	KB,V (WB/lm)
CLM-14-22-80-36-AC40	2262	0.5016	0.4184	6528	1.53E-04
CLM-14-22-90-36-AC40	2246	0.5020	0.4167	6340	1.58E-04
CLM-14-22-70-36-AC40	2239	0.5009	0.4141	5951	1.68E-04
CLM-14-24-90-36-AC40	2464	0.4813	0.4158	5041	1.98E-04
CLM-14-27-95-36-AC40	2722	0.4576	0.4094	3841	2.60E-04
CLM-14-27-80-36-AC40	2704	0.4582	0.4081	3798	2.63E-04
CXM-4-27-90-36-TC41	2666	0.4559	0.3992	3535	2.83E-04
CXM-4-27-95-36-TC41	2726	0.4515	0.3986	3422	2.92E-04
CLM-14-30-95-36-AC40	3039	0.4346	0.4041	3133	3.19E-04
CLM-14-30-80-36-AC40	3042	0.4337	0.4024	3084	3.24E-04
CLM-14-30-90-36-AC40	3049	0.4325	0.4007	2998	3.34E-04
CXM-4-30-95-36-TC41	2995	0.4326	0.3945	2941	3.40E-04
CXM-4-30-90-36-TC41	3014	0.4313	0.3940	2901	3.45E-04
CLM-14-30-80-36-AC42	3049	0.4271	0.3894	2634	3.80E-04
CLM-14-30-90-36-AC42	3045	0.4262	0.3869	2577	3.88E-04
CLM-14-30-95-36-AC42	3067	0.4249	0.3866	2564	3.90E-04
CXM-14-32-95-36-AC42	3197	0.4172	0.3846	2518	3.97E-04
CLM-14-35-95-36-AC40	3476	0.4072	0.3930	2466	4.05E-04
CLM-14-35-80-36-AC40	3441	0.4084	0.3916	2453	4.08E-04
CLM-14-35-90-36-AC40	3477	0.4068	0.3919	2429	4.12E-04
CLM-14-40-80-36-AC40	3937	0.3836	0.3798	2004	4.99E-04
CLM-14-40-95-36-AC40	3973	0.3822	0.3797	1998	5.01E-04
CLM-14-40-70-36-AC40	3969	0.3824	0.3799	1981	5.05E-04
CLM-14-50-95-36-AC40	5031	0.3446	0.3555	1533	6.52E-04
CLM-14-50-80-36-AC40	5040	0.3444	0.3557	1479	6.76E-04
CLM-14-50-70-36-AC40	5060	0.3437	0.3541	1459	6.85E-04
CLM-14-56-70-36-AC40	5574	0.3308	0.3431	1315	7.61E-04
CLM-14-56-80-36-AC40	5642	0.3292	0.3420	1300	7.69E-04
CLM-14-65-90-36-AC40	6541	0.3124	0.3267	1168	8.56E-04
CLM-14-65-95-36-AC40	6615	0.3112	0.3255	1156	8.65E-04
CLM-14-65-80-36-AC40	6515	0.3126	0.3283	1145	8.73E-04

Plot of the tabulated data. A regression line can be used to calculate KB,V from CCT.

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