1. homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesprinciples REF Physics of light REF Photometry Radiometry is the science of measuring electromagnetic radiations, including visible light. While photometry is the science of measuring the electromagnetic spectrum weighted by the visual sensibility V(λ) of the eye. So, radiometry deals with the energetic aspects while photometry deals with luminous aspects. For each aspect, four interesting emitted and received quantities can be measured.visual sensibility V(λ) of the eye In lighting design, a minimum level of illuminance is often required in a room to realize a task. Though, sensation of luminosity is measurable through luminances in the field of view and through their ratio. But, as illuminance is a quantity that can be more easily measured than luminance, lighting standards more often deal with illuminance.lighting standards Radiometry Energetic aspect Photometry Luminous aspect Emitted Radiant Power Φ e (watt - W) Luminous Flux Φ (lumen – lm) Radiant Intensity I e (watt/steradian - W/sr) Luminous Intensity I (candela- cd) RadianceL e (W/sr.m²)LuminanceL (cd/m²) Received IrradianceE e (W/m²)IlluminanceE (lux) RadiometryPhotometry K m = 683 lm/W (S.I.)

2. homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesin depth REF Physics of light Photometry 1. The luminous flux (φ– lm) is the quantity of light emitted, by a luminous source, in all directions. The unit is the lumen (lm). 2. The luminous intensity (I – cd) is the quantity of light emitted, by a punctual luminous source, in a given direction. The luminous intensity of a punctual light source is thus its luminous flux per unit solid angle in the given direction. The unit is the candela (cd). The difference between the luminous flux and the intensity is that the former is the total of all emitted light, whereas the latter is the fraction that is emitted into a certain direction or into a certain solid angle. A solid angle is a three-dimensional angle. Mathematically, it is a double integral over the longitude and latitude angles. For spherical surfaces viewed from the center, the solid angle equals the ratio of the area of the surface over the radius of the sphere [Synthlight] Architecture & Climat Some typical photometric values Luminous sourcesLuminous flux (lm)Luminous intensity (cd) Incandescent 100W1300100 Polar diagram Solid angle

3. homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesin depth REF Physics of light Photometry 3. The illuminance (E – lx) is defined as the luminous flux φ that is received per unit area of a surface. In other words, it can be described as the flux density on the receiving surface A. The unit is lux or lm/m². E = φ / A When stating the illuminance for a surface, its orientation has always to be given. For instance, the illuminance on a desk is measured as the horizontal illuminance, the illuminance on a façade or on a wall as vertical illuminance. Illuminance meters, sometimes referred as lux meters are affordable, as long as the demands on accuracy are not too high. (Synthlight) 4. The luminance (L - cd/m²) is defined as the luminous intensity I of a surface in a specific direction, divided by the projected area S as viewed from that direction. L = I/S Unless the object is perfectly diffusing, the luminance varies with the angle from which it is viewed. [Synthlight] Some typical photometric values Luminous sourcesIlluminance (lx)Luminance (cd/m²) Landscape under full moon0.20.01 – 0.1 Landscape under overcast sky5 000 – 20 000300 - 5 000 Sun in the summer100 0001 500 000 000

4. homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesapplications REF Physics of light Photometry In lighting design, repartition of light and levels of luminosity are mainly studied through the analysis of illuminance levels and luminance ratios in the field of view. Illuminance is more easily measured than luminance, that is why it is generally this photometric quantity which is referred in lighting standards. Minimal illuminance levels and mean illuminance levels will be specified according to the activity in the room. Uniformity is the ratio of minimum illuminance to average illuminance, on the work plane. It gives an idea on the luminous repartition and the presence of shadow zones in the room. Architecture & Climat

5. homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesteaching resources REF Physics of light Photometry To teach about photometry, teachers can show, in manipulating a lux meter and a luminance meter, that two surfaces can present a same illuminance but a different luminance, according to their coefficient of reflection. With the same material, he can also show that the luminance of a point of a diffuse surface does not vary whatever the angle of view. I = 200 L=0.05L=0.60