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Offline ahidayat

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measure the light level/brightness level
« on: 06/05/2009 09:14:12 »
Hi everyone,
 
i'm new here. i'm doing small research about the effect of the light to the transmitted video quality. What do i need to measure the brightness level/light level quantitatively?, can i use digital camera or web cam on my laptop to do it? is there any computer software that can do it? Would someone help me please? Thanks


 

Offline Don_1

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measure the light level/brightness level
« Reply #1 on: 06/05/2009 09:24:46 »
Any SLR camera with TTL metering will give you a light reading. Alternatively, you could get a light meter. They aren't so easy to come by these days with almost all cameras having built in meters, but the studio photographer still uses them. Most can be quite expensive (100+) but I did a quick search and found this one http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Test-Equipment/General-Testers-Meters/Digital-luminance-meter/82365/kw/85-0740?source=googleps&utm_source=googleps priced under 40.
 

Offline ahidayat

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measure the light level/brightness level
« Reply #2 on: 06/05/2009 09:58:32 »
Thanks Don,

Does TTL meter in SLR camera able to give quantitative light measure indoor as well as outdoor?(for example if i turn on two lamps in my room, it will show the light level as 100, and if i turn on only one of them it will show lower number, say 60. can it work like that?)
 

Offline Edster

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measure the light level/brightness level
« Reply #3 on: 06/05/2009 13:02:12 »
Hi can you expand what you are trying to do? I`m wondering why?
what aspect of "quality"

If you can expand I`ll try and find some useful URL`s or explain
But I`m in the dark here.

Any light meter will give the same response inside and out, what it will not tell you is the colour teperature ( unless it has that function, some meters, notably from minolta do)

Does your camera not have manual exposure? from the specification of its sensitivity, you can calculate the average light levels from the aperture when the contrast looks correct

Generally video quality from a camera will only deteriorate at low light levels due to noise in the sensors, and at high contrast ratios. The AGC on the iris which is normally the default keeps the average light input the same , but as light increases and the aperture reduces the depth of field will increase.

 In Tv  meters are usually only used for camera set up on step wedge chart, but the exposure on subject  is done by eye and the use of a waveform monitor. The lighting generally is for peak white at f3.5 minimum to get depth in a studio. ( but this depends on the production and the effects wanted)

Most meters on SLRs will be average or centre weighted, so any highlights may be clipped, as they usually  are with domestic video camera`s auto exposure systems.


 

Offline Don_1

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measure the light level/brightness level
« Reply #4 on: 07/05/2009 09:53:38 »
Metering in SLR camera's will only give you a shutter speed at a specific aperture.

For example, if you set the lens aperture to f8, your camera might give you a shutter speed of 1/125sec exposure. Keeping the camera in the same position, same focal point, same ISO and same aperture but with a reduced light level might show a shutter speed of 1/50sec. It would not give you a luminance reading. You would need a light meter for that.

With older film cameras' you will find some will have fixed shutter speeds, so the camera selects the optimum available shutter speed. Cameras' with 'programme' auto exposure are more likely to give the actual shutter speed rather than the nearest available.

TTL sensors work equally indoors and out, but in very low light levels, which the camera is unable to shoot at even at very high ISO settings, the metering may not respond.

If you require more information on the light from a specific source, you could set a digital camera to centre weighted metering, focus on the source from as close as your lens will allow, stop the lens down to the minimum aperture feasible and take a shot. For direct light you will probably need to set the camera to a very low ISO. Transfer the shot to your computer using the camera manufactures software, where you should be able to see the histogram.

Note: I don't think you can get sufficient information from software such as Microsoft, but so far as I am aware, most of the noteable camera manufacturer's (e.g. Nikon, Canon, Pentax, Fuji etc.) supply software which will show considerable detail of a shot once downloaded to your PC.
« Last Edit: 07/05/2009 09:56:25 by Don_1 »
 

Offline ahidayat

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measure the light level/brightness level
« Reply #5 on: 08/05/2009 02:38:51 »
Thanks Don,

i really appreciate your help.
 

lyner

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measure the light level/brightness level
« Reply #6 on: 08/05/2009 10:15:29 »
The camera uses the light level (averaged in some way, of course) to calculate what exposure / aperture / ISO to use. Many cameras include this information in the 'Metadata', attached to its files. This data gives you enough information to find out what actual light level is represented by a maximum level (e.g. 255, 255, 255 for the three signals) in the resulting digital picture. It would be best to use RAW or TIFF format, rather than JPEG because it won't have been messed about with by the coding of the picture data.
It depends what absolute accuracy you need but you could always calibrate your metering system by seeing the exposure for a 'good white' card in full sunlight at midday in Summer. You could hand all future measurements on that. At least they would be self consistent!.
 

Offline RD

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measure the light level/brightness level
« Reply #7 on: 08/05/2009 16:49:53 »
i'm doing small research about the effect of the light to the transmitted video quality.
 What do i need to measure the brightness level/light level quantitatively?,

An incident lightmeter which is calibrated in lux would do the job ...

http://www.sperdirect.com/cgi-bin/category.cgi?category=generalinfovisiblelight

http://www.sperdirect.com/cgi-bin/category.cgi?category=chartvisiblelight
« Last Edit: 08/05/2009 16:52:09 by RD »
 

Offline LeeE

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measure the light level/brightness level
« Reply #8 on: 08/05/2009 16:51:00 »
Hi everyone,
 
i'm new here. i'm doing small research about the effect of the light to the transmitted video quality. What do i need to measure the brightness level/light level quantitatively?, can i use digital camera or web cam on my laptop to do it? is there any computer software that can do it? Would someone help me please? Thanks

I'm afraid that the light meters fitted to cameras will not allow you to measure light levels.  Because they are directional they can only measure the level of light reflected from a scene and this will change depending on the direction from which you view the scene.  For example, consider this situation:  The the sun is to the west and there is a north-south running wall.  If you stand to the west of the wall and take a light reading off of the  with your camera you will get a high light reading.  If you stand to the east of the wall though, and then take a reading, you'll get a low value, even though the light levels haven't changed.

What you need to do is take an incident light reading, and you can only do this with a stand-alone light meter fitted with a 'cone' or a 'dome'.  These 'cones' or 'domes' are raised translucent covers that are fitted over the light sensor so that when the light meter is laid on its back, light enters them from all directions of the hemisphere.  Once the light has entered the cone/dome it is scattered within the translucent material of the cone/dome and presents an average light level of all the incident light to the light sensor.

A new light meter is very likely to be expensive but I had a look on e-bay and found a couple of Weston Master V light meters, with 'Invercones', for less than 20 UKP.  For the Weston meters, do ensure that they include an 'Invercone' as they are easily broken and might be difficult to replace now (although spares used to be easily obtained, stocks may now be running low).
 

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measure the light level/brightness level
« Reply #8 on: 08/05/2009 16:51:00 »

 

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