[Radiance-general] UDI upper limits

Tue Dec 11 13:57:59 PST 2012

John,

A very timely and useful response thank you.  As you know the EFA (Education Funding Agency) in the UK has released its FOS (Facilities Output Specification) for the next generation of schools.  The directive from the government was a higher standard of space for lower cost / m2.

We made the decision to drop daylight factors and move to CBDM for the next generation of schools and the topic of the UDI exceed level came up during the writing and testing of the baseline design and indeed it has come up again since as people try and create standard designs.  As it stands the UDI autonomous was set at 100-2000 in the FOS.  What this means for designers when you look at a balanced design, (thermal and lighting), you need to control heat gain with smaller external windows and therefore supplement the daylight from a secondary source.  However, this costs money.  E.g. Atria, lightwells, lightpipes etc.

What is interesting when you raise the UDI exceed from 2000 to 3000 then you can start again to design single sided lit spaces just as we had with DF's (indeed the type of rooms we are trying to stop building as the blinds are typically pulled down and the lights are turned on).  My thought therefore is whether in raising UDIe to 3000 because say a person can typically accept that level of light, then means we have significantly increased the contrast in a single sided lit space.  If we have, have we increased it to a level where parts of the space feel gloomy?   Is the answer to move UDIa to 300-3000, does that matter much?

Regards

Andrew
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From: John Mardaljevic [mailto:J.Mardaljevic at lboro.ac.uk]
Sent: 11 December 2012 20:27
To: radiance-general at radiance-online.org
Subject: Re: [Radiance-general] UDI upper limits

Does John M. wanna weigh in on this?

I believe Mardaljevic has been considering higher alternative thresholds as well.

Yes, on both counts.

Original UDI of 100-2,000 lux was conceived around 2004, and was, with hindsight, a little too conservative regarding the upper limit.  I suspect now that those studies giving preferred maxima less than 2,000 lux could well have been the older articles in our sample.  Quite possible that work environments of around 20 years ago were less forgiving of high daylight illuminances, e.g. curved CRT screens, etc.  It seems that the more recent studies are all nudging-up the preferred / tolerated upper limit.

Splitting the UDI range at the design level (e.g. 300 lux) was done in 2007 and applied to the (1st) VELUX study.  For the past few years I've been using the values 100, 300 and 3,000 lux to define the four UDI ranges: fell-short (<100); supplementary (100 - 300); autonomous (300 - 3,000); and, exceeded (>3,000).  For general daylighting use I don't see any need to alter the 100 and 300 lux bounds, but there may be good cause to increase the upper limit to 3,500 or 4,000 lux (greater?).  I'll be doing some sensitivity tests on that seeing how it affects annual totals for the autonomous and exceeded ranges.  It may not make much difference for many designs, at least in terms of UDI-autonomous and UDI-exceeded.  Though that's not to say that it wouldn't have knock-on consequences for, say, annual predictions of DGP.

I do find the four range UDI scheme very useful to help evaluate (and communicate) the daylighting performance of a space -- especially when there are several facade variants to be considered and a simple slide transition reveals the impact of the design change in a manner that is very readily assimilated.  For other, more specialist applications, I see no problem in adjusting the three defining lux levels to suit, e.g. 20, 50 and 200 for a gallery space where conservation is an issue.

So, UDI -- no fixed rules.  Though a consensus for general daylighting applications may be forming around the 100, 300 and 3,000 (or 4,000?) lux values.

Cheers
John

John Mardaljevic
Professor of Building Daylight Modelling
School of Civil & Building Engineering
Loughborough University
Loughborough
Leicestershire
LE11 3TU, UK

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