Light meters sensitive to IR light!

[Kirk Keyes]

"The designers of the meter may not have compensated for IR at all, or they may have calibrated to a black body spectrum, or they may have specified materials or coatings for the meter's lens or the photodiode encapsulation window that did a good job of rejecting IR. We don't know. "

I believe the ISO specification for meter calibration uses a 2850K tungsten light source the has been corrected to 4800K. I don't remember seeing any special precautions made for filtering out the IR output of the lamp.

I'm waiting for someone to come out with a multispectral light meter. It should not be diffucult to do these days, as you can buy photodiodes that have built in color filters. Check this out - http://www.taosinc.com/images/product/document/TCS230-E15.pdf "The light-to-frequency converter reads an 8 x 8 array of photodiodes. Sixteen photodiodes have blue filters, 16
photodiodes have green filters, 16 photodiodes have red filters, and 16 photodiodes are clear with no filters." It'a a tiny little IC package about 5 mm square with a 2 mm square for the diode array. Perfect for a light meter.

The neatest part of this idea I think is that this sensor puts out a signal for RGB and "Clear" - it seems to me that a combination color temp meter/ light meter could be made. Even neater, by coupling it with a bit more microprocessor power then is used in todays meters, you could program the computer to "know" the spectral response curves of several colro or B&W films (or at least film types) and then make exposure calculations based on that info. Metering through filters would be easily done and the loss in exposure easily corrected.

And as the RGB diodes also have different responses to IR (see the data sheet link), one could use an external IR filter or a pair of sensors, one IR filtered and one not, to make direct measurements of the amount of IR present and then either eliminate it from the expsoure calculation, or use the spectral response of the RGB filtered diodes in the IR region to match the filters and films that are commonly used in IR photography. You could have a #25, #87, and #87B setting for the meter or match it to your IR film type - extended red, IR or far IR.

Anyway, wish I knew enough about electronics/microprocessor programming to actually work up the design for something like this... I think a lot of people would buy a light meter/color temp meter/IR meter, with both spot and incident in the same meter, of course. I know I would.

Kirk

[bglick]

Struan, thanks for explaining... that was my exact game plan. Your previous post left me with a different understanding. Sorry. The game plan is clear, experiment with the filter, if no difference, well, I gained confidence but out $100+. If there is differences (at times), then have Richard calibrate the meter to known values of visible light.

I found that B+W makes the actual filter size that fits on my Sekonic L-608, so I will get the right size. Also, interesting data found regarding the IR filters on the Schneider site. I am curious, which IR filter is best for this purpose, one that "absorbs" IR, or one that "interferes" with IR? I can not grasp the differences as they would apply for this application.



B+W Digital UV-/IR-Blocking Filter 486 DIGITAL-PRO SLIM
This B+W Interference Filter has a completely colorless glass carrier coated with a number of extremely thin, partially reflecting layers with precisely computed thicknesses, similar to MC coating. The B+W Filter 486 does not block by means of absorption, but by interference of the unwanted UV- and IR radiation that is repeatedly reflected between these layers, affecting the wavelengths on both sides of the visible spectrum with a steep cut-off. It is used mainly on digital- and video cameras with CCD sensors without an integrated IR protection filter, because the IR sensitivity of the CCD sensor would otherwise cause color changes and unsharpness. That unsharpness results from the chromatic aberration of the lenses that are only corrected for visible light. In the visible range, the transmission curve is very high and straight. This filter is completely clear and it requires no increase in exposure. Its filter factor is 1.



B+W Infrared-Blocking Filter 489 [KG 3]
This IR-blocking filter must not be confused with heat-resisting protection filters for projectors or spotlights. Instead, it is intended for use as protection for IR-sensitive CCD sensors or in the light path of illumination devices with low thermal characteristics. It suppresses infrared radiation ≥780 nm. Because it gradually begins to absorb infrared radiation at 600 nm, long-wave red light is slightly weakened, so that this filter has a subtle green tint. But in CCD applications, this can be readily corrected electronically. Its filter factor is approximately 1.2

I am assuming the "interference" version, 486, is the better one for this application?

TYIA

[bglick]

jtf, interesting read on the Weston meter. I wish all makers would post data like this.... wow...

Kirk, your concept sounds so appealing ! Sign me up for one too! You sure are right about the technology existing for all this today. It's simply a matter of market demand, it's just not there in my opinion. The volume would be so low on such a meter, it would probably sell for $5k. I noticed even Sekonic dropped their spot meter, 778. Everything must be streamlined, one product fits all to make economic sense.

[Struan Gray]

The interference filter is better. No colour cast and a negligible absorbance across the visible spectrum.

Interference filters don't mysteriously get rid of the light they reject, they reflect it away. This one behaves like a mirror for UV and IR wavelengths and clear glass for visible light.

Kirk: nice idea, but I'd also question how much of a market exists. There are already some very nice miniture spectrometers about the size of a cigarette packet that connect to a computer via USB. These use a grating and a line-CCD rather than filters, which allows you to have more channels and/or more sensitivity. One of these with a display and a microprocessor would be nice as a colour meter, but again, expensive.

[Kirk Keyes]

"These use a grating and a line-CCD rather than filters, which allows you to have more channels and/or more sensitivity. One of these with a display and a microprocessor would be nice as a colour meter, but again, expensive."

They sure would, and the price on them is coming down. Like the Gretag Eye-One Photo, about $1400. But then the spectrometer part will most likely decrease the light sensitivity that we would need for a light meter. And we really only need simple RGB measurments to do it, not a data point every 10 nanometers. (Althought that would be cool!)

As far as the disply and microprocessor, meters like my Minolta Flashmeter VI probably already have enough CPU power and a large enough display to do exactly as I described above.

As far as the market - look how much people pay for the Zone VI digital spots, or even the new pocket light meter. They are both supposed to be filtered correctly and that is enough to make them sell. I would bet that if Minolta or Sekonic made one of these they could do it for $500-1000. Color meters are a bit above this price range, I suspect it's because of the limited market for them. The meter I propose would be the Ferrari of light meters.

It would actually be a pretty simple device - all the work would be done in programming.

Kirk

[Struan Gray]

Are you sure nothing like this exists for cinematographers? They have plenty of money, and a big incentive to get things right first time.

[Mark Woods]

We may have a lot of money but unfortunately we have to deal with the same technology the people on this forum do. If meters were only made for us, they'd be really expensive. Besides, with color negative, the latitude is far greater than chromes or well done black and white. I'd buy one of these babies too.

Kind Regards,
MW