Colour Rendering Index (CRI): CIE Colour Rendering Index (CRI) is a metric that describes the ability of a light source to reproduce the colours of various objects in comparison with an ideal or natural light source. In short, CIE CRI defines the colour rendering properties of light sources. The light sources with higher CIE CRI ratings have better colour rendering abilities. The international commission of illumination (CIE) defined the colour rendering as the “Effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant. (CIE 1987)” CRI shows the quality of light. It is measured on a scale of 1-100 Ra. A CRI of “0” is very poor (monochromatic light) and “100” is perfect light (as we see when we are outside with no artificial light). Most of the time, the artificial light sources don’t have as good Ra value as natural sunlight so the colours of an object under artificial lights appearance are different. This is one of the most critical factor in deciding the light source.

Asymmetrical Light Distribution: In the case of an elongated light with twin tubular lamps like a tubular office tube light fixture the two curves will have different shapes. Here, if you position yourself along the 0-180° axis, you can see that the upward beam is spherical (top right). The downward beam, however, is split into two spherical planes, because of the inbuilt reflector. The light is blocked by the reflector in the center.

If you look at the luminaire from the side, meaning if you position yourself along the 90-270° axis, then you can see a spherical plane upwards and downwards. This means that the luminaire distributes the light evenly. The luminaire’s elongated shape does not mean the curve is flat or elongated. The light distribution is measured from the central point of the luminaire.

Selection of Light Fittings

A luminaire, or light fitting, is a complete lighting unit. Typically it consists of a lamp or LED together with parts designed to distribute the light, position and protect the lamps, and the ballast. It also includes components to connect the luminaire to the power supply. These Luminaires can be either recessed or surface-mounted, depending on the application and mounting surface. The new age luminaires are becoming an increasingly popular choice to replace old lamps and battens as they are low-cost and relatively efficacious for replacing high-intensity discharge lamps (mostly metal halide), with T5/T8 fluorescents of now LED lights.

Lamp Wise Colour Rendering Index Ranges (Ra)
C00/1800 And C900/2700 Views                                           Symmetrical Light Distribution

Factors for Selection a Light Fixture

  • Ceiling/Suspension Height: A luminaire will over or under-illuminate an area if the height is not correct.
  • Lux Level:  This needs to be taken into account for each type of work area.
  • Open Area or Racking: If you have racking to illuminate, consider the spread of light to ensure the whole height of the racking is illuminated.
  • Colour Temperature: Colour temperature is the colour the light from the lamp looks. Warm white at around 2500K gives out a more yellow light than cool (blue) white at 6500K. Cool white also referred to as ‘daylight’, can simulate natural daylight and is said to increase productivity when used in a work environment.
  • Compatible with Existing Controls: Consider costs involved in switching out your current controls or sensors. Many, but not all, high-bay luminaires available are retro-fit and will work with existing controls or sensors.
  • Mounting Height and Position: Consider how much of the area needs to be lit. The general rule for positioning an LED floodlight is that it should be angled downwards at 22 degrees.
  • Sensor Options: LED fitting can also be procured with additional PIR sensors and controls that can detect motion, be set to remain on for a certain amount of time after detecting motion or be tuned to come on at dusk. A PIR sensor switches only when it senses movement within a given range. It is particularly useful in security lighting. The duration of light can often be set by the user. A dusk to dawn sensor is less economical than a PIR sensor.


Asymmetrical Light Distribution & Its Polar Diagrams

  • Luminaire Size: Typically sizes of 600 x 600mm or 1200 x 600mm are available.
  • Unified Glare Rating (UGR): Unified Glare Rating (UGR) is a psychological measure of Discomfort Glare in interior lighting applications. It can be calculated precisely for a specific lighting arrangement. UGR is defined by document CIE 117-1995, Discomfort Glare in Interior Lighting, and clarified in terms of the tabular display in CIE 190:2010, Calculation and Presentation of Unified Glare Rating Tables for Indoor Lighting Luminaires. For applications where visual display equipment is used, panels with UGR of <19 are usually recommended.
  • Product Fire Rating: TP (A) and TP (B) are terms used in the UK Building Regulations (UK Building Regulations, Fire Safety, Approved Document B, and Volume 2 – Buildings other than dwellings)  & classify lighting diffusers according to their flammability. TP means thermoplastic, and TP materials, such as polycarbonate (PC), acrylic (PMMA) and polystyrene (PS), are commonly used as diffusers in light fittings. While TP (A) rated thermoplastic diffusers do not have any restrictions (except protected stairways and exits enclosed with fire-resisting construction) and can therefore be used in all areas with no limitation, TP (B) rated thermoplastic diffusers are restricted to a maximum of 5m2 for a group of recessed panels with a minimum 3m space between each group of panels.
  • Colour Temperature: Colour temperature is the colour the light from the lamp looks. Warm white at around 2500K gives out a more yellow light than cool (blue) white at 6500K. Cool white, also referred to as ‘daylight’, can simulate natural day light and is said to increase productivity when used in a work environment.
  • Dimming: Unlike traditional lighting, dimming with LED is more complex but with the right guidance you will achieve the dimming functions you need. It is important, however, to seek expert advice.
  • Control Gear: Depending on fittings one may need to fit new control gear. Generally, any retrofit panel should work with the existing control gear.
  • Indoor or Outdoor Application: It is important to check the IP rating of fittings based on their application. IP65 is best for any outdoor application.
  • Protection from Harmonics: With the increasing use of LEDs which generate a relatively high level of harmonic distortion in current waveforms, necessary precautions must be built in the system form these harmonics. For example, a 25 W LED lamp that complies with the IEC 61000-3-2 standard and injects 85% of third harmonic and 61% of fifth harmonic, obtains a Total Harmonics Distortion of 105%, which is extremely high.


“In the beginning there was nothing. God said, ‘Let there be light!’ And there was light. There was still nothing, but you could see it a whole lot better.” Was probably said as a joke but ‘The Light’ as a phenomenon is a much more complex subject than it appears. Many detailed books have already been written, while many researches have been done over the time yet none of them can be said to explain this complex phenomenon of light in totality.

However, these studies have helped humans understand the light in order to perceive the world around them and even beyond that even in outer space. Yes, light is amazing because it’s not just about what is visible but its impact is visible in areas that are unseen like aesthetics, our ability to perform tasks as well as our mood.


Nature And Nature’s Laws Lay Hid In Night, God Said Let Newton Be! And All Was Light.” Alexander Pope’s I 1688

Prabhat Khare possesses a BE (Electrical) degree from IIT Roorkee (Gold Medalist). Now, he is the Director of KK Consultants.

Leave a Reply