Luminance vs. Illuminance
Luminance and illuminance sound like similar terms in the lighting world but have vastly different meanings. Find out the difference here. This means that the colors are always perceived in relationship to the colors around them, .. have the same “brightness” if exposed to the same illumination. The Exercise two directly tests your understanding of these terms and concepts . It is really very useful for you to do these quick quizzes. Once you have finished white page. Luminance is measured in candelas per square meter. (cd/m2).
Because the exposure depends upon the time during which the light acts on the film, the film is able to integrate the quantity of light falling upon it.Surface Illuminance
A practical advantage of this effect is that through sufficiently long exposure it is possible to photograph objects which might otherwise not be sufficiently bright to produce a photographic image. The exposure is not the only factor determining the photographic effect produced, although it is a very important factor in this connection. The photographic effect, by which is meant the density of the silver deposit, is determined by the characteristics of the sensitive material and by the processing conditions as well as by the exposure.
These factors are related graphically by means of the D log10E characteristic and form a major component of the topic of Photographic Sensitometry.
By means of the D log10E characteristic curves, it is possible to determine the density produced on a certain photosensitive material for given exposure and processing conditions.
Luminance and Illuminance
Such curves provide a clue to what might be expected, by way of the photographic effect, when photographing an object with a camera and lens system.
However, the D-log10E characteristics are usually expressed in terms of density and meter-candle-seconds. The illumination of the object being photographed is not ordinarily determined in meter-candle-seconds, and even if this were possible, through the use of properly calibrated exposure or illumination meters, the intensity of illumination on the plate is vastly different from that of the or iginal object because of the reduction in size, the effect of the aperture stop, the focal length, and other characteristics of the lens system.
To make maximum use of the sensitometric concepts and to understand fully the various and numerous factors which enter into exposure, it is desirable to provide the connecting link which relates the exposure, as given in the sensitometric sense of the term, and the brightness of the object as this may be determined by measurements with an exposure or illumination meter.
It is proposed to construct this connecting link based upon theoretical considerations for two reasons: Optical system of a camera showing axial rays. The luminous intensityI', of the point P' on the photographic plate can be expressed in terms of the luminous intensity, I, of the point on the subject, P, and the characteristics of the lens system.
Image Brightness as a Function of Optical System We will now establish the connecting link by which the brightness of the image on the photographic plate may be determined from the illumination of the original object being photographed.
This link involves the optical system of the camera, which, so far exposure is concerned, includes the iris diaphragm, the bellows extension, a filter if one is usedand the shutter, as well as the lens system proper. The iris diaphragm or aperture is represented as being at A. The principal planes of the lens are represented as lying at PP and at PP', while the entrance and exit pupils are designated as being at NP and XP, respectively, and the principal focal lengths are L and L'.
The point P may be self-luminous or may be illuminated by reflected light. In either case it will illuminate the entrance pupil of the lens NP with an intensity inversely proportional to the square of the distance between P and R, the latter being in the plane of the entrance pupil, and directly proportional to its luminous intensity I.
Let the distance between P and R be X. The distance X may be considered as being made up of two components.
lighting quiz 1 Flashcards Example for Free
So when you tilt an illuminated lambertian surface, the brightness remains equal regardless of angle. Try that with your favorite lambertian surface by tilting it, with a fixed source. When the source hits the part you observe at a small angle, the brightness goes down. Perhaps you are thinking about the effect of equal brightness. Then the cosines cancel but this has nothing to do with the falling light, which is fixed for that effect.
Going back to the OP, those are two different quantities, and you cannot "convert" one into the other. One has angular dependence, the other one does not. This is essential for photography since the solid angle is determined by the aperture.
In the same way, you cannot convert seconds to meters but you can tell the distance traveled over an interval of time if you now the velocity. That would not be a "conversion".
It would be an answer to a specific question. Relation, but not a conversion is discussed here: This answers the question about the total luminance of a surface over all angles given the illuminance. The luminance itself not the total one is a different quantity not comparable to the illuminace. My favorite is Barium Sulfate, and is actually the industry standard.
Sure, but tilting the surface with a stationary source changes the illuminance of the incoming light relative to the surface, which changes the reflected luminance by the same amount, so once again the 1 to 1 relationship holds independent of angle.
If instead you keep the source fixed to the surface so they move together and change the angle of your luminance measurement, the fall off you describe only occurs at extreame angles for well made reflectance standards, and is a function of surface finish only.
One can get very consistent results between luminance and illuminance for angles between 10 and deg where it is angke independent again. No the falling light doesn't have to have to be fixed.
As I mentioned, changing the angle, changes the illuminance, and the measured luminance follows. It's rather humerous how you claim that you can't convert one into another, then site a link below where they do just that, using the special case that I mentioned originally, that you tried to dispel.
That's a straight conversion sir.
You should know better. Measured luminance is independent of area. Given a uniformly illuminated surface, if I measure the luminance of 1 square cm, or of 1 square meter, I get the same value.
How can there be "total luminance". The only reason they integrate is to get the intensity, which is how they relate it to the illuminance.