QUALITY OF LIGHT
- COLOUR TEMPERATURE
Colour temperature is a description of the warmth or coolness of a light source. When a piece of metal is heated, so called ‘black body’, the colour of light it emits will change. This colour begins as red in appearance and graduates to orange,yellow,white,and then blue-white to deeper colours of blue. The temperature of this metal is a physical measure in degrees Kelvin or absolute temperature. While lamps other than incandescent do not exactly mimic the output of this piece of metal, we utilise the correlated colour temperature (or Kelvin’s) to describe the appearance of that light source as it relates to the appearance of the piece of metal (specifically a black body radiator).
By convention, yellow-red colours (like the flames of a fire) are considered warm, and blue-green colours (like light from an overcast sky) are considered cool. Confusingly,higher Kelvin temperatures (3600-5500 K) are considered warm. Cool light is preferred for visual tasks because it produces higher contrast than warm light. Warm light is preferred for living spaces because it is more flattering to skin tones and clothing. A colour temperature of 2700-3600 K is generally recommended for most indoor general and task lighting applications. Colour Temperature is not an indicator of lamp heat.
2.COLOUR RENDERING INDEX
CRI,or Colour Rendering Index, is a measurement of a light source’s accuracy in rendering different colours when compared to a reference light source with the same correlated colour temperature. It generally ranges from 0 for a source like a low-pressure sodium vapour lamp, which is monochromatic, to 100,for a source like an incandescent light bulb,which emits essentially blackbody radiation. The higher the CRI,the better the visual perception of colors. CRI is related to colour temperature, in that the CRI measures for a pair of light sources can only be compared if they have the same colour temperature. The higher attainable CRI is 100.Lamps with CRIs above 70 are typically used in office and living environment. A standard “cool white” fluorescent lamp will have a CRI near 62.A standard “cool white” led has a CRI of 75 and “warm white”.80-85.
3. COLOUR CONSISTENCY
Colour consistency refers to the average amount of variation in chromaticity among a batch of supposedly identical lamp samples. Generally speaking, the more complicated the physics and chemistry of the light source, the more difficult it is to manufacture with consistent colour properties. This is why consistency is a problem for some light sources, particularly metal halide and LEDS.Different samples from the same batch may exhibit different chromaticities. To limit this variation, the lightening industry uses a colour consistency based on MacAdam ellipses.It was found by MacAdam that all homogeneous colour matches made by an observer fell into an ellipse on the CIE 1931 chromaticity diagram. The measurements were made at 25 points on the Chromaticity diagram, and it was found that the size and orientation of the ellipses on the diagram varied widely depending on the test colour. These 25 ellipses measured by MacAdam, for an observer are shown on the Chromaticity diagram. So a homogeneous colour impression results in an elliptical area within the CIE 1931 (x,y)-chromaticity diagram, the so called MacAdam ellipse with size 1 SDCM (or 1 STEP).
CLASSIFICATION OF FIXTURES
- ELECTRICAL SAFETY
The electrical safety classification drawn up by the IEC embraces 3 main fixture classes. Class I,II and III.They can be summarised as follows:
Fixtures in this class, apart from being electrically insulated, are also provided with an earthing point (labelled)connecting all those exposed metal parts that could conceivably become live in the presence of a fault condition. Where the luminaire is provided with a flexible power lead, this must include an earth wire. Where this is not the case, the degree of electrical protection afforded by the luminaire is the same as that afforded by one of Class 0.Where a connection block is employed instead of a power lead, the metal housing must be connected to the earth terminal on the block. The provision made for earthing the luminaire must in all other respects satisfy the requirements for Class I.
Class II fixtures are so designed and constructed that exposed metal parts cannot become live. This can be achieved by means of either reinforced or a double insulation, there being no provision for protective earthing.In this case of a luminaire provided with an earth contact as an aid to lamp starting, but where this earth is not connected to exposed metal parts, the luminaire is nevertheless regarded as being of Class II.A luminaire having double or reinforced insulation and provided with an earth connection or earth contact must be regarded as a Class I luminaire.However,where the earth wire passes through luminaire as part of the provisions for though-writing the installation, and is electrically insulated from the luminaire using Class II insulation, then the luminaire remains Class II.
The fixtures in this class are those in which protection against electric shock relies on supply at Safety Extra-Low voltage (SELV),and in which voltages higher than those of SELV (50 V AC r.m.s.) are not generated. An AC operating voltage age of 42 volt maximum is common. A Class III luminaire should not be provided with a means for protective earthing
2. DUST AND MOISTURE PROTECTION (IP CLASSIFICATION)
The IP (International Protection ) system drawn up by the IEC classifies fixtures according to the degree of protection afforded against the ingress of foreign bodies, dust and moisture. The term foreign bodies includes such things as tools and fingers coming into contact with live parts. The designation to indicate the degrees of protection consists of the characteristics letters IP followed by two numerals indicating conformity with the condition stated in two tables. The first of these so-called ’characteristic numerals’ is an indication of the protection against the ingress of foreign bodies and dust, while the second numeral indicates the degree of sealing against the penetration of water.