Design of a photopic detector

Methods for V(λ) correction

A very precise spectral match of the photometer head to the standard luminosity function V(λ) of the human eye is necessary for accurate photometers. There are a number of different  approaches for achieving V(λ) correction. Full filtering is the method of choice. This entails attaching multiple slices of various color filter glasses to the silicon photodiode. Partial filtering is an alternative method and this involves using different small filter glass particles to cover parts of the light-sensitive area. Both methods allow V(λ) correction in conformity with the requirements of DIN 5032-7 class L where f1’ ≤ 1.5%. The full-filtering method is more common because it keeps the error index f9 low. This error results from non-uniform illumination of the detector area and can be significant, particularly in the case of light sources with high gradients (e.g. headlamps).

Figure 1: a) Example of full filtering, b) Example of partial filtering


Figure 1: a) Example of full filtering, b) Example of partial filtering

Cosine correction

The illuminance on a surface changes with the cosine of the angle of incidence relative to the perpendicular to this surface. Since a regular photodetector does not fulfill this requirement cosine correction needs to be carried out for a photometer. A diffuser and suitable shading at the detector edges are used to correctly capture incident light coming from all directions. However, photometer heads which are deployed together with a stray light tube for taking measurements in the far field do not require any cosine correction.

Figure 2: Diagram showing cosine correction

Figure 2: Diagram showing cosine correction

Classification of illuminance measuring instruments in conformity with DIN 5032-7 and EN 13032-1 (2004)

Class of measuring instrument
CharacteristicLABC
V (λ) correction f11.5 %3 %6 %9 %
UV sensitivity u0.2 %1 %2 % 4 %
IR sensitivity r0.2 %1 %2 %4 % 
Cosine response f2-1.5 %3 %6 %
Linearity error f30.2 % 1 %2 %5 %
Error of the display unit f40.2 %3 %4.5 %7.5 %
Non-uniform illumination f915 %20 %30 %40 %
Calibration error f110.1 %0.5 %1 %2 %
Total characteristic (without f9) ftotal3 %5 %10 %20 %