Photopic Filter

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This filter simulates the photopic response of the human eye to relatively bright light. In the plot below, the circles indicate the design targets -- the desired transmission of the filter.

Plot of photopic filter performance

This design is for normal incidence on a BK7 (ordinary) glass substrate. Two materials are used: SiO2 and TiO2. Starting with a thin, one-layer design, TFCalc's needle/tunneling method produced a sequence of 11 designs of increasing complexity. The design (whose performance is shown above) consists of 32 layers. It matches the entire photopic response curve to within 1%. Phil Baumeister notes that the design can be improved (or simplified) by using a glass whose transmission cuts off near 700 nm.

Here is the design, with the first layer closest to the substrate and thickness given in nm: 

    SIO2     119.32
    TIO2      10.09
    SIO2      32.61
    TIO2      79.19
    SIO2      19.85
    TIO2       9.73
    SIO2     118.20
    TIO2      87.90
    SIO2     128.88
    TIO2      82.48
    SIO2     121.93
    TIO2      83.91
    SIO2     112.35
    TIO2      89.57
    SIO2      92.26
    TIO2      66.01
    SIO2       8.09
    TIO2      21.45
    SIO2     103.04
    TIO2      59.41
    SIO2      32.45
    TIO2      15.36
    SIO2      98.82
    TIO2      51.98
    SIO2      77.54
    TIO2      28.26
    SIO2      68.84
    TIO2      49.47
    SIO2      84.44
    TIO2      43.02
    SIO2      53.15
    TIO2      39.34
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