My hand-held lightmeters don't read right in incident mode ...

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xpatUSA
Members 1496 posts

May 16, 2026, 7:05 p.m. May 16, 2026, 7:05 p.m.
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By which I mean that the recommended exposure is low compared to that derived from a commercial meter used for determining building lighting requirements.

For example, my ebay BTMETER BT-5000A reads about 430 lux under my desk lamp, compared to my recently-acquired Sekonic non-selenium L-398A's 860 lux at the same distance.

The reason is that the Sekonic constant C (340) is not a calibration constant per se. It is a value deduced from a large sample of test shots for Sekonic models. For a Sekonic, the exposure value recommendation is EV=log2(E.S/C) ... where E = illuminance in lux, S = ISO. So, the Sekonic recommended 8 EV Exposure Value.

But the BTMETER read 430 lx, i.e. 40 fc - for the which needle position, the Sekonic would have recommended 7 EV, double the exposure.
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xpatUSA
Members 1496 posts
May 17, 2026, 2:53 p.m. May 17, 2026, 2:53 p.m.
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@xpatUSA has written:

By which I mean that the recommended exposure is low compared to that derived from a commercial meter used for determining building lighting requirements.

For example, my ebay BTMETER BT-5000A reads about 430 lux under my desk lamp, compared to my recently-acquired Sekonic non-selenium L-398A's 860 lux at the same distance.

The reason is that the Sekonic constant C (340) is not a calibration constant per se. It is a value deduced from a large sample of test shots for Sekonic models. For a Sekonic, the exposure value recommendation is EV=log2(E.S/C) ... where E = illuminance in lux, S = ISO. So, the Sekonic recommended 8 EV Exposure Value.

But the BTMETER read 430 lx, i.e. 40 fc - for the which needle position, the Sekonic would have recommended 7 EV, double the exposure.

More:

ChatGPT has written:

Lux meters are calibrated to the human visual response curve:

V(λ)

But camera sensors and photographic films do not perfectly match that curve.

So under unusual lighting e.g.:

[b]LEDs,[/b] sodium vapor, fluorescent lamps, RGB stage lighting,

a lux reading may differ significantly from what a camera exposure system would be based upon.
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xpatUSA
Members 1496 posts
May 17, 2026, 4:30 p.m. May 17, 2026, 4:30 p.m.
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@xpatUSA has written:

@xpatUSA has written:

By which I mean that the recommended exposure is low compared to that derived from a commercial meter used for determining building lighting requirements.

For example, my ebay BTMETER BT-5000A reads about 430 lux under my desk lamp, compared to my recently-acquired Sekonic non-selenium L-398A's 860 lux at the same distance.

The reason is that the Sekonic constant C (340) is not a calibration constant per se. It is a value deduced from a large sample of test shots for Sekonic models. For a Sekonic, the exposure value recommendation is EV=log2(E.S/C) ... where E = illuminance in lux, S = ISO. So, the Sekonic recommended 8 EV Exposure Value.

But the BTMETER read 430 lx, i.e. 40 fc - for the which needle position, the Sekonic would have recommended 7 EV, double the exposure.

More:

ChatGPT has written:

Lux meters are calibrated to the human visual response curve:

V(λ)

But camera sensors and photographic films do not perfectly match that curve.

So under unusual lighting e.g.:

[b]LEDs,[/b] sodium vapor, fluorescent lamps, RGB stage lighting,

a lux reading may differ significantly from what a camera exposure system would be based upon.

FWIW here's the spectral response of an amorphous Silicon light-sensor:

www.researchgate.net/figure/Measured-spectral-response-of-a-single-amorphous-silicon-PSD-element-from-the-array-shown_fig2_221895892
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xpatUSA
Members 1496 posts

May 19, 2026, 11:03 p.m. May 19, 2026, 11:03 p.m.
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@xpatUSA has written:

By which I mean that the recommended exposure is low compared to that derived from a commercial meter used for determining building lighting requirements.

For example, my ebay BTMETER BT-5000A reads about 430 lux under my desk lamp, compared to my recently-acquired Sekonic non-selenium L-398A's 860 lux at the same distance.

To clarify, the Sekonic meter actually indicates footcandles which I multiplied by 10.764 to convert to 860 lux.