# Lumen measurment using a cameras light meter?

#### ericmark

Most of us have a camera with built in light meter, but we do not have a lux meter. I am sure there is a way to use a camera to measure the light from a bulb, but I would guess the bulb would need putting in a white box or camera aimed at a grey card or something similar.

Colour of walls in a room, size of room, where the light points to will all have an effect. But not really wanting an accurate measurement just an approximate way to measure light so we have some reference point when talking about lighting. Easy enough to convert aperture, speed and ISO into an exposure value (EV), but it's the where to point the camera.

Going back to basics The Bunsen photometer came to mind.

I cribbed this from via Google from http://www.solitaryroad.com/c1003.html to save typing it

The Bunsen, or grease spot, photometer. The Bunsen photometer, sometimes called the grease spot photometer, consists of a white sheet of paper with a translucent grease spot in its center placed on a meter stick between a standard lamp and a lamp of unknown candlepower. See Fig. 4. The paper is moved back and forth along the meter stick until it is equally illuminated on both sides. At this point the grease spot practically disappears. We then measure the distance r1 from the Standard lamp to the screen and the distance r2 from the lamp of unknown intensity to the screen and substitute into above. I1 is the intensity of the standard lamp and we solve for I2.

Joly, or paraffin-block, photometer. The Joly, or paraffin-block photometer consists of two paraffin blocks such as one can buy in a grocery store separated by a thin sheet of metal (e.g. tin foil). In a darkened room set a standard lamp and a lamp of unknown candlepower about five feet apart and hold the double paraffin block photometer between them. The light from either side is transmitted by the paraffin but is stopped by the metal. One observes the edges of the blocks. The edge of one block will be brighter than the other and by adjusting the position of the photometer one finds a position in which both sides are of equal brightness. The distances to the blocks are then measured and the calculations are made as with a Bunsen photometer.

Reading that it would seem we could compare any 360° light source with another by simply noting camera settings and distance from lamp then moving camera closer or further away from unknown light until the readings are the same so U = unknown and K = know so LumenU = LumenK * DistanceU²/DistanceK².

It would need some white translucence material held in front of the camera lens to insure a general rather than a spot reading. The slide copier does just that.
It just happens I have one.

So for me I can set camera to manual set the meter to mid zero position and simply measure distance. No good in day light but at night one readings are made I could calibrate the slide copier lens to be a lux meter. Am I reading that correct?

Most of us have a camera with built in light meter, but we do not have a lux meter. I am sure there is a way to use a camera to measure the light from a bulb, but I would guess the bulb would need putting in a white box or camera aimed at a grey card or something similar.
By definition, illuminance (i.e. that measured in lux) relates to the illumination of a surface. You would therefore have to point your camera, or other light meter, at a (preferably 'perfect white') surface at a particular distance from the light source. I presume that a problem in relation to your intended application is that the illumination of a surface is not an effective measure of the light output of the source, since it will depend upon the distribution of the light. Hence, if you undertook your experiment in a room in which everything other than your white surface was 'perfect black' (so that there would be no 'secondary illumination' of your surface by reflection), a light source of a given light output with a small 'beam angle' would result in much more illumination (lux) of your test surface than would a source with the same light output with a much larger 'beam angle'.

Do you not need to be pragmatic, in terms of real-world situations? Since the purpose of lighting is to illuminate things of interest (worktops, tables, newspapers, walls or whatever), should you not just point your camera/meter at those 'things of interest'.

Another possible complication is that, as you will know, illuminance (lux, lumen per m²) takes into account the sensitivity of the human eye to differing wavelengths, whereas 'irradiance' (measured in W/m²) does not - and I don't know where a camera's light measurement stands in relation to those two extremes (I presume that depends upon the sensitivity of the camera's sensor or film to different wavelengths).

Kind Regards, John

The standard camera has three filters red, green and blue so I would expect the meter which uses the same sensor as takes the picture is quite similar to human eye the bayer array has twice as many green as red and blue sensor filters to compensate and make it closer to human eye. You can get black and white cameras which don't have the filter and can also use inferred but these are rare.

I like many others I am sure have swapped to so called energy saving lighting and I have made some errors, lucky I got cheap bulbs which could be moved around so swapping where the bulb is used allowed me to correct errors. I have also made mistakes working out what is bright and what is dim I see red light as being dimmer than blue light and only when I sit down to read do I realise the lighting is not good enough.

So I think the A4 sheet of white paper and camera set so it fills the screen is likely best method. It will not give lumen but will allow me to adjust lighting in each area to give enough light to read.

So I think the A4 sheet of white paper and camera set so it fills the screen is likely best method. It will not give lumen but will allow me to adjust lighting in each area to give enough light to read.
Indeed. That's essentially the 'pragmatic' approach I was suggesting. How much illuminance (lux) you get from a lamp of a particular light output (lumens) will obviously depend on many factors (associated with both the lamp and the room) but, in the final analysis, it is the illumination of surfaces in 'places that matter' (e.g. where you will be reading) that actually matters.

Whether using you camera and a white surface will give you a better answer than simply determining how easy you find it to read is perhaps a different matter!

Kind Regards, John

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