Light meters sensitive to IR light!

I was always curious how sensitive Light meters are to IR light. I took my Sekonic meters and subjected them to IR light from a night vision scope. Quite to my surprise, both the ambient and the reflective meter was quite sensitive to IR light. Of course, the lower the light levels, the more sensitive it was. At low light levels, say 2 EV, the IR would drive up to 7 EV. At higher light levels, 8 EV, the light meter was influenced between .5 and 1 stop. Of course the night vision scope puts out small amounts of IR light.

Half the suns energy is released as IR energy. The earths atmospher blocks most of the IR light. However, I am curious just how much of the IR is blocked? Is there specific times when the IR levels are higher then others? Certain places? If there was a simple solution, such as using an IR filter over the light meter, however, this is nearly impossible from what I have learned. Possibly another solution would be to use a standard IR filter used for IR film, as if you cut out all visible light, possibly the meter will read an infrared value??

I realize the amounts that may or may not effect the light meter are probably small, but with chrome film every bit helps. Any experience or research data on this subject? TYIA.

As you noted, Bill, the atmosphere acts as a variable ND filter for IR. The more atmosphere the sun's rays go through, the less IR. Thus, season (the angle of the sun) and time of day both affect the level of IR present.

I've heard, but not verified, that many light meters incorporate an IR filter over the sensor, as do some digital cameras, to block IR and reduce any exposure reading error for conventional films.

Silicone cells are very IR sensitive. Film see light to a value of around 650nm. Different films have different cut off points 650nm is a good average for most films color and black and white. Where a visible light photocell peaks in the range of 700 to 800nm, which is in the red range going into the IR range out side of what films see. Where you would notice this problem the most would be late daylight reflecting off green leaves. The light at that time of day is in the red - orange range and the leave will filter out what little blue – green light there is. Zone VI took care of this problem with their modification.

As Richard says, this is one of the good things about the Zone VI modified Pentax spotmeters. No IR.

Well, its obvious, the latest Sekonic spot and dual meters which I own, both do not have IR protection built in. So I assume, many do not. If one wants to protect from this possible mis reading, I would assume it would be best to use the same Zone VI filter which they install. Any tips on how to accomplish this task?

Also, has anyone ever done some real world testing on just how much the IR effects readings in the field?

In the 'near IR' (that's the band you're talking about), the atmosphere is quite transparent except some partial absorption near 900nm. Modern light meters, those with silicon diodes, don't see past about 1050nm. As mentioned, these peak at about 850nm. BW film 'sees' best on the blue end of the spectrum so it is quite the opposite of the silicon diode responsivity. For proper metering, a well-designed filter for the visible and an IR cutoff filter are required. Without these, your meter can be tricked with strong filters or subjects with strong coloration (including 'colors' in the near IR).

I don't know of any testing of all meters, nor do I know which meters have IR cutoff filters. Seems as though there was an article in Photo Tech about IR sensitivity years ago. IR cutoff filters are common in video cameras and I suppose in the digital cameras. The only light meters that I know of that have a shaped light band just for BW are the Z6 Pentax meter and the Pocket Spot light meter.

It should be noted that not all films are the same. Tech Pan was very red sensitive. Ilford films see green better than Kodak. T-grain films have more red sensitivity than standard grain.

It seems as if every corner you turn in this field, there is more un resolved issues. It amazes me after 150 years how more of this information is not better documented, specially by the companies who make light meters or light meter modifications. After reading a very thorough post by Paul Butzi regarding practical differences between the modified Zone VI meter (which block IR light) vs. non modified meters, one would conclude, in the real world this is not a problem. Which leads to the question, why hasn't Zone VI published any data regarding real world situations whereas IR sensitivities can have an undesireable effect on exposure?

One poster mentioned that 10,000 Zone VI meters have been sold. Of course this by itself is not evidence the modifications are solving a problem that exist in real world photography. Many photographers, including myself, would just pay the small price difference in hopes it may save an exposure or two. Of course if the modification costs several thousand dollars, people would not so easily buy based on hearsay. Very few of us will spend the time Paul Butzi did to discover just what differences really exist after buying the meter. Of course, Paul was loud and clear about the fact the modifications had no negative effects on exposure either.

Struan pointed out, that hazy conditions are prone to IR penetration moreso then visible light. This was actualy the reason I made this post. When I look back at the few exposure errors I have made, they mostly are on overcast days. I am not sure if this is a result of higher levels of IR vs. Visible, color temp sensitivity of the light meter, or coincidental times of mishaps. Of course my error is always underexposed color chromes, which would lead me to beleive Struan may have found the culprit. (Geee, what a surprise, huh) From what I can tell, I don't think Paul tested the hazy overcast condition. Maybe hazy conditions reflecting off green foliage? Hopefully Paul or others will ring in on this...

A meter will be in 'error' any time that the IR light in the scene is stronger than its designers and builders assumed. Mist or haze is complicated because it mixes scattered light with direct light in a non-trivial way, so I would be cautious about a general prescription. That said, IR does penetrate water-vapour mist and fog better than visible light, so foliage in sunlight behind a fog bank would be a good test of how badly your meter can be fooled. You would need to make sure that the fog bank itself was in shadow.

You can buy interference filters with a hard near-IR (and UV) cutoff. They won't work well for wide-field imaging, but would be useful as an add-on for a spotmeter. Not cheap though. Edmund Optics sell them as "UV/IR-cut" filters in standard metal mounts:

http://www.edmundoptics.com/onlinecatalog/DisplayProduct.cfm?productid=1524

Ah, finding this thread explains some of the email that's dropped into my mailbox in the last few hours.

Without having done more than a moment's thinking about it, I'm not convinced that adding IR cutoff filter to a meter which has none is a great idea.

As Struan points out, without a cutoff filter, the meter will be in error whenever the IR in the scene is more prevalent than the meter designers assumed. Likewise, it will ALSO be in error whenever the IR is LESS prevalent than the meter designers assumed.

So just whacking out the IR does not necessarily solve the problem.

Even worse, the designers assumptions will be right only for one film. They might be very, very close for a whole slew of films. But, for instance, if you have film with extended IR sensitivity (like Kodak's IR films, or Maco's, or the Ilford SFX, or the Konica stuff that maybe isn't made anymore) then the meter will not work right, because the spectral response of the meter won't match that of the film.

God knows what the IR response of digital cameras is like. I assume they have IR cutoff built in, or they'd never get accurate color.

Struan, can you explain this in more detail....

> They won't work well for wide-field imaging, but would be useful as an add-on for a spotmeter.

Now, based on many generous contributions, I would like to propose the following, before I do it! I buy the UV / IR cutoff filter from Edmunds. I mount on my spot portion of the Sekonic 608. An adapter will be required. Now, to compensate for what the "designers had in mind", after installing the filter, I send the meter to Richard Ritter and have him re calibrate it, using non IR light source.

Then I assume I have a pretty safe system, except, if the IR present in the atmosphere exceeds the filters cut out limit. But I am going to assume this will be rare, or possibly never? Yeah? No?

If this seems feasible, then I will surely have some good data in due time to report back, as I wil constantly check the differences with / without the filter under all types of lighting conditions. TYIA