Daylighting Collaborative

tools > ask the experts > q&a archive

Answers to your daylighting questions

Q. I am an architectural 3D renderer trying to use real world light values in an interior rendering. I am using a rectangular light in a 4 x 4 window to mimic daylight coming through and want to know if there is a lumens/sf value (or range) that I can use for an unobstructed north facing window on a cloudless day at noon.

—C.L., Salt Lake City, UT, USA

A. I ran an AGi32 model of a 20' x 20' space in Salt Lake City, Utah with a clear (100% VT) north-facing window at noon on the summer solstice. The model predicts 1426 lumens/sf at the window surface. You can probably assume that these results are reduced linearly as the visible transmittance (VT) of the window is reduced. Hope that helps.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. Are you familiar with Mass CHPS Criteria? Specifically EQ.P7.1? Is the maximum qualifying area based upon the view window area limited to 5' of ht (2.5' above and 7.5' below the floor)?

—J.L., Boston, MA, USA

A. I am not particularly familiar with the Massachusetts CHPS Criteria. However, I read through the relevant section and it appears that your understanding is correct. The minimum and maximum height limitations are trying to address the issue that a window doesn't have much of a view if you can only see the ground or the sky through it. Views needs to be natural and dynamic in order to have a positive effect on one's psyche. I hope that helps.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. I have a small project and need to know how to transmit the sunlight via fiber optic cable. It can only transmit the laser light. Could you please show me how to do it or do I need any device?

—L.H., Vietnam

A. I'm sorry, but this isn't my area of expertise. The system you are describing sounds like a hybrid daylighting system. Oak Ridge National Laboratory ( developed this technology and the California Lighting Technology Center conducted some follow up research ( You could try contacting either of them.

Our October 2008 newsletter (pdf) describes the technology in more depth. However, it does not address your problem specifically. You may also consider trying:

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. I have a shop area approximately 6450 sqft, currently lit with 26 metal halides (400W each). How much would it cost to install day lighting in the shop area?

—N.G., Fairfax, VA, USA

A. The cost of installing skylights and photosensor controls of electric lights will vary from project to project. However, I can give you a range of costs associated with daylighting. The Department of Energy commissioned a study on toplighting feasibility that pointed to a cost of $1.25/ft^2 (no light well) to $4.70/ft^2 (with a light well). This assumed a 3-stepped dimming control strategy.

These numbers work out to a cost for your project of between $8,000 and $30,000. This cost will be lower if you are already planning to replace the roof anyway. I created a quick SkyCalc model that shows approximately $1,500 of annual energy savings for making this upgrade. Therefore, your payback should be atleast 5 years. However, there are often rebates associated with this type of technology. Check out what's available in your area.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. I have an interior space where traditional daylighting methods would apply. I read an article from Oct 2008 that talked about remote source lighting using fiber optics. Are there any reasonable residential solutions?

—J.K., Burlingame, CA, USA

A. I don't have much experience with these systems. However, I do know that they use parabolic mirrors to collect sunlight and transmit it into the space via fiberoptic cables. The light is then introduced to the space via hybrid lighting fixtures that use the daylight when it is available and electric light when it is not. The advantage of this systems over more traditional skylights is that they can get daylight to area of the building not adjacent to the roof or walls.

My understanding is that these systems are rather expensive and not particularly wide spread. They make more sense on commercial buildings because the electric lighting required is coincident with the daylight being provided. In a residential setting, the electric lights are needed predominantly in the morning and evening when there isn't as much daylight to be used.

You could look into tubular daylight devices. These devices utilize reflective tubes to bounce the light deeper into the building, through attics and around corners. Please visit our directory of tubular device manufacturers.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. I am working on a solar daylighting system as my final year project. I am using an optical lens for concentrating sunlight. I want to theoretically calculate the temperature of the concentrated sunlight at the focal spot of the lens to ensure that it does not melt the optical fibres. I am unable to find a mathematical formulation for this purpose. The mathematical model that I developed is showing an excess of 1000 degrees celsius than what I experimentally measured.

—S.P., Chennai, India

A. I do not know the extent of your heat transfer experience or software access. However, if you have access to Maple and Engineering Equation Solver (EES), you can modify this example for your purposes. If not, it should at least serve as a decent guide on how to analyze this type of problem.

The main difference between it and your problem is that your lens is focusing the incident radiation, while the example lens is not. You can ignore this difference, but to make your estimate more accurate you will need to develop a different equation for the volumetric generation term. If this term becomes too complex, you will need to solve this problem numerically.

This example came from the following book if you would like more information: Nellis, G., and Klein, S., "Heat Transfer," Cambridge University Press, 2009.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. How do daylighting strategies differ between certified green building (LEED) and non-certified green? What is the best architectural contribution of daylighting in LEED?

—E.M.H., Johor Bharu, Malaysia

A. Most of my experience with certified green buildings is with the LEED rating system. So, with respect to LEED, there are many LEED certified buildings that employ no daylighting controls, as well as many buildings that are not certified at any level that do.

LEED does not require any particular daylighting strategy to be employed. However, the LEED rating system does foster an integrated approach to design, bringing the architects, engineers, lighting designers, commissioning agent and the rest of the design team together early in design as well as more frequently throughout design. Studies (for example Sidelighting Controls Field Study) have shown that this integrated approach to design results in more effective daylighting systems. LEED further requires a minimum amount of commissioning be pursued. Rubinstein et al. has shown that properly commissioning a daylighting system is necessary to ensure that its assumed energy savings is realized. In this way, LEED is promoting effective daylighting systems.

With respect to the architectural contribution of daylighting in LEED: there are really only two credits that deal directly with this (See our LEED Guide page, Credit 8.1 and 8.2). Combined, these credits push architects to design buildings with a large proportion of square footage having perimeter access. They further promote tall window head heights with high visible transmittance glazing to bring the most daylight as deep into the spaces as possible.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin

Q. I am working on a daylighting feasibility study for a St. Paul-based steel fabrication facility. The client's facility includes a shop that is in need of roof repair. The owners asked me to look into energy saving options including installing fiberglass panels in the shop roof for daylighting. I would appreciate any leads or advice on how to proceed.

—D.W., Minneapolis, MN, USA

A. Toplighting sounds like a great idea for this project, particularly in light of the fact that they are repairing their roof anyway. I don't know of specific contractors in the St. Paul area. However, any of these manufacturers should be able to assist you in that regard.

I would further recommend using SkyCalc (a free, excel-based spreadsheet) to quickly analyze a daylight design for the fabrication facility. It's easy to use and allows you to play around with the skylight properties as well as the number of skylights until the light levels on the working plane are in line with your design requirements. You can download SkyCalc for free from Energy Design Resources.

Please also see our directory of skylight manufacturers.

—Scott Schuetter, PE, LEED AP BD+C, Energy Center of Wisconsin