For many years F&A have specialised in mechanical and electrical contracting for commercial projects. F&A have partnered some of the most celebrated architects and designers and the leading design & build contractors in the industry. F&A have seen the incredible change in trends over the years in all aspects of M&E. As with any specialism, there will always be advances and changes. One particular specialism that is no different is lighting. Everyone has a different opinion about the best practises and standards. One of the most argued and debated is UGR. Aidan Twomey (F&A Pre-Contracts manager) explains…

One common question we are asked over and over “is that light fitting UGR compliant?”. This is something that lighting manufacturers stamp on packaging and a common misconception within the design industry. UGR stands for Unified Glare Ratio which in the basic of terms identifies how bright a light fitting appear in-situ. This is not a new term and dates back to the earliest of lighting codes.

Many years ago lighting in offices was a difficult task when computer manufacturers used highly reflective glass CRT screens. The obvious reflections caused huge problems. However, this was easily worked around shortly after by computer manufacturers by introducing low-reflective screens or specialised screen covers. Job done. Around this time the infamous abbreviation “Cat 2” was born.

The “Cat 2” fluorescent luminaire had a reflector with a 65 degree cut off. From here on in the abbreviation “Cat 2” defined all interior lighting and angered the SLL (Society of Light and Lighting) so much that the category system was scrapped in 2002.

Since 2002 the SLL have released Lighting Guide 3 and most recently Lighting Guide 7. Compliance with these guides requires an in depth understanding and serious consideration to illuminances, surface reflectance, task areas and more. After years of battling, “Cat 2” luminaires were forgotten and no longer produced (conveniently) and UGR 19 was born. All of sudden, like clockwork UGR 19 luminaires were being manufactured. THERE IS NO SUCH THING AS A UGR 19 COMPLIANT FITTING.

There are of course luminaires that form part of a UGR 19 compliant scheme but the UGR isn’t calculated in a lab in Germany, it is specific to the individual environment in which the luminaire is placed. Compliance can only be proven once the metrics of how the luminaire relates to occupancy in a real space has been established.

The equation to understand and calculate UGR is detailed below. You will see from the equation how specific the equation is and the many variables that are required to calculate UGR.

UGR = 8 log [0.25/Lb Æ©(L2ω/p2)]

L Represents the luminance value of the luminaire.
Lb is the value of the background luminance.
ω is the solid angle of luminaire as observed by the viewer.
p is The Guth index. This is a factor based on the likelihood of glare, known as Visual Comfort Probability.
Æ means that ALL fittings within the space are included within the equation.

LED lighting with its large illuminance levels (sometimes in excess of 4000lm on an LED panel) delivers an excess of light direct to the eye. This returns us to the era prior to “Cat 2” however using much brighter fittings. This is glare.

There are many ways to reduce glare within offices. Please give us a call if this is something you believe you have an issue with and F&A will be able to assist.

Aidan Twomey

Author Aidan Twomey

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