Explaining the Color Rendering Index (CRI) and LEDs

Microsoft Word - bae_3872_revised manuscriptCRI, or the Color Rending Index, refers to the quantitative measure of a light source’s ability to render known colors of materials as well as objects. The process entails measuring the light emanating from a test source and monitoring the appearance of reflected light as compared to 8 known color samples. The CRI is usually rated on a scale ranging from 0 to 100; the minimum index represents a light whose colors aren’t distinct while the latter corresponds to daylight or an ‘ideal’ white light.

Calculating CRI

The CRI of that particular source is calculated by measuring the difference between the actual color & the known color. The nature of the reference light source will also vary depending on the kind of light source that’s being tested. When it comes to testing sources featuring a correlated color temperature that’s less than 5000 0K (degrees Kelvin), it’s advisable that a blackbody radiator is utilized as the reference source. When dealing with test sources featuring a higher CCT, you should use a spectrum of daylight as the reference source. Regardless of the light source being tested, it’s always important to pick the right reference source so as to retrieve accurate CRI information from the sample colors.

For many decades, the Color Rending Index (CRI) scale has been considered as an accurate measure of how effective lighting is in reproducing expected colors through the spectrum. Regardless of which lighting type is being utilized, it’s essential that you measure the quantity of light before and after installation.

Unified Glare Rating (UGR) explained

Unified Glare Rating (abbreviated as UGR) is a measure of the amount of discomfort emitted by luminaries using a psychometric-based scale of discomfort. In other countries, UGR is used to represent VCP (an abbreviation for visual comfort probability). Glare, which can adversely affect your wellbeing as well as productivity at work, is common in settings featuring several computer monitors.

The Unified Glare Rating starts from 5 and goes all the way to 40. The lower numbers indicate less glare while the higher ones signify more glare. The UGR is usually calculated using the following strict formula:



  • Lb represents the background luminance in cd/m2
  • L represents the luminance of each part in the observer’s direction eye (cd/m2);
  • w represents the solid angle of the luminous parts( i.e. of each luminaire and it should be at the eye of the observer. It’s given in sr
  • r represents each luminaire’s Guth position index (i.e. the displacement from the sight line)

Factors that affect the UGR value

UGR 1When determining the Unified Glare Rating, factors such as measurement point, reflection, the location, as well as the lighting product operation are considered.

Quality classifications

UGR 3According to the UGR classification, there are 5 different quality based classes. They include 16, 19, 22, 25 and 28. 28 represents the maximum UGR value permissible in specific spaces. It’s also important to note that a luminaire featuring a UGR value of lower than 10 results in zero glare.

Summary of UGR values

  • Technical drawing requires a UGR value of less than16
  • Offices should have a UGR of less than 19
  • Reception areas should have a UGR value of less than 22
  • Archives, stairs as well as lifts should have a UGR value of less than 25
  • Corridors and passageways should have a UGR value of less than 28



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