Radiance and Reflectionolor Theory

 img1.jpgLuminous Flux Light emitted from a source or received by a surface is referred         ———————- to as luminous flux, which is measured in Flux in Lumens (lm).
 img11.jpgMeasurement of Luminance When measuring the reflected light from a surface it will depend upon a combination of the nature of the surface finish and its reflectance and will change with the viewing angle. The brightness of the-surface is measured in candelas per metre square (cd/m2) using a luminance meter as shown
 img11.jpgMeasurement of Illuminance Illuminance meter T-10A is capable of accurately measuring next generation lamps including PWM controlled lighting. Easy multi-point measurements (2 to 30 points). Simple and accurate illuminance measurements.
 img11.jpgMeasurement of Colour Rendering & Colour Temperature (Spectrophometer)When measuring the reflected light from a surface it will depend upon a combination of the nature of the surface finish and its reflectance and will change with the viewing angle. The brightness of the surface is measured in candelas per metre square (cd/m2) using a luminance meter as shown.

 

img5.jpgCossine Law of Illuminance If the surface is turned so that rays strike at an angle, the illuminated area will increase in size and the illuminance will drop accordingly. The ration of the original illuminated area to the new area is equal to the cosine of the angel through which the surface has been moved. Therefore, the illuminance will fall by a factor of the cosine of the angle. This is the cosine law of illuminance. If a surface illuminated to 250 lux is twisted through an angle of 60 degrees, then the illuminance will fall to half or 125 lux because the cosine of 60 degrees is ½.
img7.jpgReflectance When light strikes an opaque surface ? and by opaque we mean a surface that will not transmit light ? some of the light is absorbed and some reflected. The ratio of the luminous flux reflected to the luminous flux received is know as reflectance. If a small element of the surface receives 100 lumens and reflects 70 lumens, then the reflectance is 0.7 ? or it can be expressed as a percentage as 70%. The remaining 0.3 or 30% would be absorbed.
img8.jpgDiffuse Reflectance Different surfaces also reflect light in different ways. For example, surfaces such as paper, emulsion paint, carpets and so on, exhibit what we call matt or diffuse reflection ? that is, the light reflected from the surface is scattered in all directions.
 img9.jpgSpecular Reflections At the other extreme is mirror or specular reflection exhibited by shiny metal surfaces such as chrome silver or pure aluminium. It is most important to realise that although specular reflections produce a clear image in the surface of the material, the actual amount of light reflected may be deceptively low. A matt white painted surface, for instance, has a reflectance of 85% to 90% compared with only 60% specular reflectance from a polished stainless steel surface, while polished aluminium will be approximately 85%.
 img10.jpgMixed Refletions Many surfaces such as gloss paint, wood, plastic and so on, exhibit a combination of these two types of reflection. Gloss paint, for example scatters most of the light that it reflects, but also produces a specular reflection in the surface of the paint.
 img2.jpgLuminous Intensity Luminous intensity is a measure of how much flux is emitted within a small conical angle and is expressed in Candelas. The inverse square law states that the illuminance E=l/d2, the intensity of the light source divided by the distance squared.