Question

I think that the safety will be mitigated if the source of light is wide, not the point light. Would you please tell me the calculation of MPE (Maximum Permissible Exposure) and the emission power in such a case?

Answer

(1) Mitigation of the safety in the case that the source of light is wide, not the point light

The safety is mitigated if the visual angle of the source of light is more than 1.5mrad and the wave length is 400〜1400nm because the image on the retina is enlarged according to the visual angle and the power density is reduced.

＊ visual angle : the angle of the source of light from the measurement point

For example the visual angle is 0.05rad if the lamp of 10cmφ is 2m apart. The safety is increased according to the ratio between the visual angle of the source of light and 1.5mrad. In the above case the safety is about 33 times compared to the source of light of the visual angle 1.5mrad.

＊ The reason that the minimum visual angle is 1.5mrad is as follows. The minimum image on the retina is about 30μm because of the movement of the eye and the aberration of the lens, and the diameter of the eye is 23〜24mm. The value of 1.5mrad is used as the minimum visual angle.

Now the visual angle of the source of light corresponds to the angle of the image on the retina from the lens one to one.

(2) Calculation of MPE (Maximum Permissible Exposure) and the emission power

1. MPE

    

   MPE is defined based on the physiological information obtained from the animal experiment. MPE is 10% value of the exposure power where the probability of the biological damage is 50% and the probability of damage is very small even if we are exposed by the light of MPE level.

    

2. Emission power

    

   MPE is obtained from IEC60825-1 Table 6. Measurement is performed based on the diameter and the distance shown in IEC60825-1 Table 10. In the case of outdoor equipment the distance is 2m and the diameter is 50mm because the telescope is probably used. If the measurement result is more than MPE, it is not included in class 1.

＊ IEC60825-1 can be obtained from IEC Web site.    http://www.iec.ch/