Candela, Lumen, Lux: the equations
The candela (unit cd) has its origin in the brightness of a "standard candle", See the section on lux for the relation between candela and lux. What is the relationship between the illumination and the energy of light? power and luminous intensity depends on the color of the light according to the luminosity function: Related Questions (More Answers Below). This means that the colors are always perceived in relationship to the colors around them, .. Luminance may also refer to the amount of light passing through a .. some problems, because the interior illuminance might actually have a direct.
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.3. Physics - Photometry - Relation Between Luminous intensity & Illuminance - by Ashish Arora
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'.
Luminance and Illuminance | Physics Forums
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.
Luminance and Illuminance
Strange term, and odd concept, however once you understand what it really is, things start to make more sense. To take a step back, lets talk about radians first. A radian is an angular measure, which simply put means the following: One radian is an arc that is equal in length to the radius of a circle.
A radian is measured in the two-dimensional plane. A steradian is similar to a radian, only measured in three dimensions.
Candela, Lumen, Lux: the equations
The definition of a steradian is as follows: One steradian is a circular patch on a sphere's surface who's area is equal to the square of the radius of the sphere. A steradian is an odd "projection" of a 2D subtended angle into three dimensional space, or what is called a solid angle. The intersection of the 2D angle with the sphere's surface intersects a circular patch which is itself bisected by the radian arc.
Another term for it is squared radian. The candela measures the amount of light emitted in the range of a three-dimensional angular span.
Since the luminous intensity is described in terms of an angle, the distance at which you measure this intensity is irrelevant. For ease of illustration, in the picture at the right the three dimensions have been flattened to two. In this picture, screen B would catch exactly the same amount of light rays emitted from the light source as screen A —provided that screen A were removed to not obscure screen B. This is because screen B covers the same angle as screen A.
The angular span for candela is expressed in steradian, a measure without unit like radian for angles in a two-dimensional space.
exposure - What is the difference between luminance and illuminance? - Photography Stack Exchange
One steradian on a sphere with a radius of one metre gives a surface of one m2. See the section on lux for the relation between candela and lux. Lumen If you look at LEDs, especially high-brightness LEDs, you may notice that the LEDs with a high luminous intensity in candela or milli-candela, mcd typically have a narrow apex angle.
Similarly, LEDs with a wide apex angle typically have a relatively low luminous intensity. The same is true for halogen spots with reflector: The cause for this relation is the total energy produced by the LED. However, when a LED emits its total energy in a beam with a narrow angle, the intensity will be greater in the direction of that angle than when the same energy had been emitted over a wide angle.
The lumen unit lm gives the total luminous flux of a light source by multiplying the intensity in candela by the angular span over which the light is emitted.