Talk:Light meter

message: 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)
thread_title:  illumination use 

message: 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)
thread_title: Style consistency

message: 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)
thread_title:  Please expand 

message: How does one actually measure lumens?-69.87.204.232 12:37, 19 May 2007 (UTC)
replies: text: Here we're really more concerned with illuminance per unit area (lux in the SI) than with the total output (lumens in the SI). The measurement of the output of luminaires (where the total output of interest) is discussed briefly in Photometry_(optics). JeffConrad (talk) 04:34, 2 October 2010 (UTC)
thread_title:  Lumens 

message: 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:
* 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)
thread_title:  Light level chart 

message: 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."
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."
-69.87.204.232 13:51, 19 May 2007 (UTC)
thread_title:  Plant growing light meters 

message: Industrial & Commercial Grade Precision Light Meters, including Footcandle Meters, Lux Meters and UV (UltraViolet) Meters $50-150
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
-69.87.203.79 17:31, 19 May 2007 (UTC)
thread_title:  General purpose 

message: 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)
thread_title:  Consistency of incident-light metering 

message: 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. , , . Han-Kwang (t) 15:21, 10 October 2007 (UTC)
replies: text: 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:   Ssat = 78 / Hsat, where Hsat  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 Snoise40 =  10 / HS/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)
replies: text: 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)
replies: text: I propose we continue this discussion on Talk:Film speed. Han-Kwang (t) 11:26, 12 October 2007 (UTC)
thread_title:  Headroom in digital sensitivity 

message: 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:
# We imply that most meters are well matched to the spectral sensitivities of most films.
# 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)
thread_title:  Spectral Sensitivity 

message: The mention of an integrating sphere isn't really correct, for two reasons:
# An incident-light meter can have a flat rather than hemispherical reflector.
# The function of the hemispherical receptor on a meter isn't quite the same as that of an integrating sphere, and the link serves more to mislead than educate.
The function of the receptor isn't important in this context; what matter is the incident-light measurement's independence of the reflectance of scene elements. The function of the hemispherical receptor is adequately (and accurately) explained in the section Calibration constants.
I'm going to remove this sentence unless someone has a good reason for keeping it. JeffConrad (talk) 04:52, 9 February 2009 (UTC)
replies: text: Edit : Discussion moved to "angle of acceptance" topic - Redbobblehat (talk) 20:29, 9 February 2009 (UTC)

text: My point was that reference to an “integrating sphere” is at best superfluous. Norwood's description of the hemispherical receptor is the best that I have found, but it obviously isn't a solid mathematical treatment. If a more rigorous treatment exists, it would be an excellent addition to the article. JeffConrad (talk) 15:55, 9 February 2009 (UTC)
replies: text:  I accept that “integrating sphere” may not be the best analogy. In the interests of 'preserving the phenomena' would something like "a diffuser with a 180° angle of acceptance" be a better substitute ? It strikes me that "180°" is not strictly accurate, but  "2π steradian" is inappropriate to the idiom ... would "hemispherical angle of acceptance" be intelligible enough ? or "a diffuser with a hemispherical field of view" ? --Redbobblehat (talk) 20:29, 9 February 2009 (UTC)
replies: text: I agree, "integrating sphere" is a different sort of animal altogether, a large, expensive device for measuring lumens of an emitter (or reflectivity of a material). I think "a diffuser with a hemispherical field of view" is wordy but more descriptive. I am making this change. Erich666 (talk) 21:30, 7 July 2022 (UTC)
thread_title:  Integrating sphere 

message: I have learned that Thomas Edison, the inventor of photography, invented the light meter too. Unfortunately, I've got no reliable references, can anyone help?  --141.91.129.2 (talk) 09:32, 21 August 2009 (UTC)
replies: text: There are many reliable sources that can clear up your idea that he invented photography! Dicklyon (talk) 00:25, 22 August 2009 (UTC)
replies: text: It is commonly known that Edison invented it. My parents told me so, do you suggest they were liars????  —Preceding unsigned comment added by 84.141.34.227 (talk) 08:11, 23 August 2009 (UTC)
thread_title:  Who intented it? 

message: There remains perpetual confusion on many photography blogs regarding the above. I think the section Light_Meter#Calibrated_reflectance answers it in part (and yet they still argue). I think it might be easier for lay readers to get their heads around it if we explain that by converting the image into a jpeg or tiff - the noise is simply removed - and this creates a pseudo Ev reduction.  Thus making it appear that the light-meter (either built-in or hand-held) is calibrated to ≈18%. The footnote on page 11 of  The ISO Definition of the Dynamic Range of a Digital Still Camera,
Douglas  Kerr makes this easy to understand when explaining noise removal– without the math.
Quote:
 “It is this margin provision that causes the confusion between the roles of an 18% average reflectance and a 13% average scene reflectance as premises for the “standard” exposure meter calibration; the luminance that is 18% of “70.1% of maximum recordable luminance” is 13% of the maximum recordable luminance itself.”
However, this Wikipedia  article  is about light meters only and not cameras and what happens to the RAW image data. So I'm not sure how to add this information to the article. I have just looked at the Sekonic L-208 TwinMate specs on their site. They quote: C = 340  and  K = 12·5.  So, (π*12.5)/340 = 0·1155 or 11·6% if you like.  Well within the ISO specifications. Yet converted a RAW file exposed with the help of this instrument to jpeg or tiff and a pseudo change in mid tone will appear as if the camera was calibrated to about 18% (give or take 1/3 of a stop (as allowed in the ISO std for jpegs and tiffs).  Have also found a sketch of how to use a grey card properly. . One is aiming for diffuse reflection  not specular reflection (which the often quoted angle of 45° will encourage). Also, the card will appear about 1/3 stop dimmer.  That together with the  ≈ -1/3 stop most digital cameras appear to meter, from a grey card full-on and the Ev will be about right for most shots.  I'm hopeless at drawing but if you think it would be useful here and on Grey card  I will issue a request on WP for a drawing to be done. After all, the idea itself is not copyrightable although this image is.--Aspro (talk) 02:25, 6 December 2014 (UTC)
thread_title: Confusion between Mid Reflectance Value vis 18% Mid Tone (geometric)  Value