Talk:Light meter

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Contents 1 illumination use 2 Exposure determination with a neutral test card 3 Style consistency 4 Please expand 5 Lumens 6 Light level chart 7 Plant growing light meters 8 General purpose 9 Consistency of incident-light metering 10 Headroom in digital sensitivity 11 Spectral Sensitivity

[edit] illumination use

I added illumination use, because light meters are very often used in automatic regulation of lighting.

There is, however, a question of combining different articles related to that: 'PIN diode' or 'light regulation', which is currently, quite unappropriately, redirected to 'Dark-sky movement' article

Mitjaprelovsek 16:56, 29 April 2006 (UTC)

[edit] Exposure determination with a neutral test card

I added the math to this section in attempt to show the basis for readings obtained with a neutral test card. Almost as this was added, however, it struck me as adding more clutter than utility. Unless someone objects, I'm inclined to remove it. Thoughts? JeffConrad 22:41, 15 October 2006 (UTC)

It's part of the article that people will skim-read, so it's not important. But what does "The instructions also recommend that the test card be held vertically and faced in a direction midway between the Sun and the camera" mean? I cannot picture this in my head. -Ashley Pomeroy 22:41, 16 March 2007 (UTC)

Example: imagine an outdoor scene in which the Sun is 30° to the left of the camera. The Kodak instructions would have the test card held vertically, with the perpendicular to its surface 15° to the left of the camera. In my experience, this doesn't always work as expected. The primary problem seems to be that a typical test card is far from a perfect diffuser, so that specular reflections can sometimes have significant effect on meter readings. But the instructions are what they are, so that is how I described them.

No one has objected to the deletion of the math, so I assume that I probably should not have included it in the first place. JeffConrad 00:24, 17 March 2007 (UTC)

[edit] Style consistency

A minor note: the remainder of the article is in American English, so I think "analog" might be preferable to "analogue". There are far greater issues, though; this article has had many contributors, and it reads as a bit of a hodgepodge. It would benefit considerably from an overall edit. JeffConrad 00:24, 17 March 2007 (UTC)

[edit] Please expand

This article could use a lot of work. Much more needed about the history of light meters. How well the various sensors/instruments cover how wide a range of lighting levels. Typical costs. Much more about non-photography uses of light meters, in lighting design and test, scientific uses. Links to such sources.-69.87.204.232 12:37, 19 May 2007 (UTC)

[edit] Lumens

How does one actually measure lumens?-69.87.204.232 12:37, 19 May 2007 (UTC)

[edit] Light level chart

Lux is the unit of measure most often used to measure light. (The USA still measure in foot-candle*, 2500lux = 26900 foot-candle). Here are some typical lux figures: [1]

• moonlight - <1lux
• living room indoors at night (in front of the TV) - 50lux
• well lit kitchen or office - 500lux
• turn your car lights on in the middle of the day so other people see you coming - 1000lux
• MINIMUM to treat SAD (typically in 2hours) - 2500lux
• clear spring morning, 30 minutes after sunrise in UK (also High Speed 30minute SAD treatment light)- 10,000lux
• bright summers day in UK - 50,000lux
• a high mountain in Kenya (on the equator) at midday - >100,000lux

-69.87.204.232 13:39, 19 May 2007 (UTC)

[edit] Plant growing light meters

Plant growing light meters are readily available, starting at about $20 USD online:

Hydrofarm Light Meter
"This Hydrofarm Plant Light Intensity Meter is an easy and accurate way to measure natural, fluorescent, or HID light for gardening use. With a range from 0 - 5000 foot-candles you can monitor your lamp's intensity for seasonal light changes in your home. HF's Light Meter is permanently calibrated, accurate to 2%, and requires no batteries. Also includes a booklet with optimum footcandles for a variety of plants." [2] [3]

Rapitest Moisture/Light Meter
"Manufacturer: Luster Leaf (#1830) UPC: 035307018304 Retail: $19.99
The ultimate tool for measuring two of the most important elements for healthy plants - light and water. Instantly switches from moisture to light meter. No batteries required." [4] -69.87.204.232 13:51, 19 May 2007 (UTC)

[edit] General purpose

Industrial & Commercial Grade Precision Light Meters, including Footcandle Meters, Lux Meters and UV (UltraViolet) Meters $50-150 [5]

LX-1010B
Economic Lux meter ever made $40 USD Specifications: To check the level of bright Range:0- 2,000/20,000/ 50,000 Lux (+-5%+2D) Accuracy: 5.0% Resolution: 1 Lux Sampling time:0.4 second Dimensions: Body: 4.6 X 2.7 X 1.10" Sensor: 3.26 X 2 X0.8" Weight : 160 g. Measure 0 -- 50,000 lux for a wide range of use. Ideal for use by architectures, light designers, and photographers.. Weight: (Lbs) 0.36LB [6] -69.87.203.79 17:31, 19 May 2007 (UTC)

International Light made scientific radiometers, and they have now merged with Gilway, in Massachusetts US. Specialized units called "belt monitors" are sold for monitoring UV curing facilities. This page has a number of detailed radiometer detector spectral curves, from 200-2000 nm.[7] It would be great to have details about photo detectors of various types in WP.-69.87.203.9 11:55, 30 May 2007 (UTC)

[edit] Consistency of incident-light metering

Is it right to remove the comment about the consistency of incident-light measurements? Although I question whether they are more precise (or more "correct"), I think they usually are more consistent than reflected-light measurements of a given scene. It might be helpful to hear from a cinematographer on this. JeffConrad 21:09, 26 May 2007 (UTC)

[edit] Headroom in digital sensitivity

According to this article, the ISO film speed S is given by S=Kn^2/Lt, where L is the luminance of a medium-brightness area (candela/m^2), n the aperture number, K a constant (usually between 12 and 14), and t is the shutter speed. Usually, medium brightness is taken to be an 18% reflective card. However, this doesn't say anything about the headroom. Should a sensor with a given ISO number be at exactly 18% of saturation for a given luminance L? Or is there a standard amount of headroom? I've searched a bit and I found numbers of 106%, 170%, and 141% for the saturation level. What does the ISO document say about it? This information seems to be missing. See e.g. [8], [9], [10]. Han-Kwang (t) 15:21, 10 October 2007 (UTC)

Solving for S doesn't mean the meter is telling you that that's a definition of S, or a recommendation for S. I mean, yes, given a luminance and a meter calibration, etc., you can back-solve for S. But there's nothing there that says the L value is taken from a "medium-brightness area", is there? It's from wherever you aim the meter. Now, there is actually a saturation-based digital camera ISO speed definition that works something like you say, but it has its own constant, not dependent on any particular meter calibration of course, and leaves about a half-stop of headroom above a diffuse white when average scene is something 18%, if I recall correctly. But nobody rates their cameras by this definition anyway, so don't expect to find any such relationship in practice relative to any ISO setting or rating on a digital camera. If you want to get technical ISO 12232 says "The saturation based speed, Ssat, of an electronic still picture camera is defined as: S_sat = 78 / H_sat, where H_sat is the minimum focal plane exposure, in lux seconds, that produces the maximum valid (not clipped or bloomed) camera output signal." This may not be trivial to interpret as you want, but compare this to the noise-based ISO definitions of the form S_noise40 = 10 / H_S/N40 where that H level is in some sense a "mid gray"; the ratio of saturation to this mid-gray is thus 7.8, which is more than 1/0.18 by a factor 1.404, or about a half stop. I hope this helps. Dicklyon 18:53, 10 October 2007 (UTC)

I see, so the equation n^2/t = LS/K is a simply a definition of what the reading that a light meter should produce for a given luminance, even though K is somewhat variable. But as a photographer, of course I would want to know how much headroom I will get if I point the light meter at a 18% card. As you say, officially the saturation level should be 141% (1/2 stop above 100%), but in practice YMMV (e.g. my digital compact camera seems to aim for 18/100 rather than 18/141). Maybe this subject really belongs on film speed, where I also asked this question. Anyway, I think the focal plane exposure is given by H=pi L t/4 n^2 (for objects far away from the lens), so Ssat = 78/Hs = 312 n^2/ (pi*Lsat*t) where Lsat is the luminosity that just saturates the sensor. I guess that 78 comes from 100*pi/4, so Ssat=100 n^2/(Lsat*t), which seems to be an elegant equation -- would it be appropriate to mention on film speed? Han-Kwang (t) 23:43, 10 October 2007 (UTC)

I've posted something on Talk: film speed. Now that I have read Dicklyon's posts here I see that my post there is a re-write of the end of his previous post - ie that the 78 factor in the Ssat formula comes from 141/18. The formula for focal plane exposure used in 12232 (Eq 2) takes flare exposure (Hf), cos^4 loss, lens transmission factor (T) and vignettng factor (v) into account. The pi/4 combines with the other losses (T v cos^4theta, with Hf << H) to produce 65/100 (which is 0.83 pi / 4), so the formula for arithmetic mean focal plane exposure is 65 La t / (100 A^2) where La is the arithmetic mean scene luminance and A is your N. On axis the focal plane exposure would be 0.88 pi L t / (4 A^2). Helen Bach 22:47, 11 October 2007 (UTC)

I propose we continue this discussion on Talk:Film speed. Han-Kwang (t) 11:26, 12 October 2007 (UTC)

[edit] Spectral Sensitivity

I wonder if the paragraph on spectral sensitivity (near the bottom of the section Use in photography) isn't a bit misleading, for two reasons:

1. We imply that most meters are well matched to the spectral sensitivities of most films.
2. We imply that correction of a mismatch is usually straightforward.

What constitutes a “good match” is somewhat subjective; if it means essentially that most meters recommend reasonably acceptable exposures most of the the time, then the match probably is good in most cases. But various meters have wildly differing spectral responses. For example, Pentax spotmeters have very broad response, extending from slightly into UV to well into IR; Kenko (formerly Minolta) have rather narrow response, approximately that of the 1931 CIE photopic observer. In many cases, reasonable results can be obtained with either meter, although relative readings change a bit when measuring objects of different colors. In extreme situations, the differences can be astounding. For example, in a yellow-light room used for semiconductor photolithography, I have seen indications by Pentax and Minolta meters differ by 5 EV; presumably, at least one was incorrect ...

Correcting the response by filtration may work quite well on a meter with broad spectral response, but is less effective on a meter whose built-in filtration already attenuates the response in the region of interest for a particular film.

Much time and effort has been expended in numerous forums arguing about differences in nominal calibration among various manufacturers and about alleged “18% calibration” vs. “12% calibration,” but either difference is often small in comparison with differences in spectral sensitivity. JeffConrad (talk) 19:58, 6 February 2008 (UTC)