Light Diffusion Power

 

Light Diffusing Power (LDP) is a new measurement and rating system that allows building designers to objectively compare light diffusing ability of various glazings.
The example below illustrates LDP and its impact on daylighting and light levels.

ldp_room

Direct sun strikes glazing at 45°

and enters an idealized space

+

 

 

Radiance Renderings:
The Renderings below show the daylight pattern for glazings with various LDP’s

 
 

ldp_translucent_clear_glass

Clear Glass LDP: 0.00 – Non-diffusing

 

ldp_translucent_acid etched

Acid Etched Glass LDP: 0.03 – Poor

 

ldp_solera

Solera LDP: 0.95 – Excellent

Graphical LDP:
 

ClearGlass

Clear Glass

 

AcidEtched

Acid Etched Glass

 

Solera

Solera

Explanation:

Light diffusing glazings are powerful elements for designers who wish to bring high quality natural light into interior spaces. Translucent glazings vary greatly in their ability to diffuse light, and selecting the right glazing can be critical to the success of a project. However there is no simple rating system that allows a designer to simply compare the performance of light diffusing glazings at a glance. Full characterization of a translucent glazing results in a large multidimensional set of data that is too complicated for anything but an optics expert or a computer simulation program. On the other hand, simple numbers like ASTM’s ‘haze’ are not designed for this purpose and cannot differentiate between between common glazings with very different light diffusing abilities, such as Solera vs. acid etched glass.

Advanced Glazings Ltd. created ‘Light Diffusing Power’ to address this critical need. LDP has three forms: a simple graph, a number from 0.00-1.00, and a Descriptive rating [non-diffusing,
poor, moderate, good, excellent]. The images above show computer simulations of daylight patterns in a very simple space, when glazings with various LDP’s are used. The wide variation in LDP is obvious, and the impact on daylight quality is striking.

Objectives:

 

LDP was designed to meet the following objectives:
a) It must clearly and simply differentiate glazings with various levels of light diffusion, in a manner that is relevant and useful to building designers.
b) It must be technically sound.
c) It must be objectively measured by simple techniques, and ideally it should be simple enough that any technically competent individual with a spotlight and light meter could verify claims.
d) It must break new ground in terms of relevance to real world conditions. Most glazing optical properties are measured with incident light at normal incidence, despite that fact that most glazing is vertical and it is very rare for light to strike a glazing at such an angle. Therefore, to be more relevant, LDP is to be measured at a somewhat arbitrary non-normal angle, chosen to be 45°. This angle was chosen as a compromise, since higher latitudes or winter conditions would have a generally greater angle of incidence and lower latitudes or summer conditions, a lesser angle.

Setup:
ldp_experimental_setup_graphic-sm
Measurement:
The glazing sample is illuminated by a quartz halogen spotlight, at an incidence of 45 degrees. A light meter measures the radiant intensity leaving the aperture in the mask that covers the back of the sample. The measurement is repeated over a range of angles from -85 to +85 and the results are recorded. (Measurements at exactly 90° not physically possible)
Graphical LDP:
The detector readings are divided by cosine of the angle to account for variation of apparent aperture width with angle, and the results are scaled such that the peak reading is unity. A plot of this data is the graphical LDP.
Numerical Ratings:
Note that in real daylighting situations, LDP values for positive detector angles represent daylight that is redirected upwards off the floor, where it will strike ceiling or walls resulting in superior lighting. Thus a numerical rating is calculated that represents the percentage of transmitted light that is redirected to positive angles.
Descriptive Ratings:
Descriptive ratings are assigned to various numerical LDP’s according to the table below:
 
Descriptive
LDP
Numerical LDP
Non-Diffusing
0
Poor
0-.20
Moderate
.20-.50
Good
.50-.80
Excellent
.80+
 
 

Sample LDP data for various glazings:

Unit Configuration
Numerical LDP
Descriptive LDP

The LDP presented in the table to the left was determined by numerically integrating the curves in the chart below. Double the area from 0-90° divided by the total area under the curve yields this number. A perfect scattering diffuser would yield a 1.00.

Physically what this number represents is the amount of light that is redirected to the walls and ceiling, instead of the floor.

Acid Etched
0.003
Poor
White PVB
0.25
Moderate
 
401-401
0.30
300-300
0.73
Good
300-545
0.91
Excellent
545-545
0.95

 

ldp_comparison_graph
Limitations and Comments:
Various simplifications and arbitrary choices were made in order to meet the objectives outlined above. As a result:
1. LDP is not applicable to glazings that diffuse light in a manner that is asymmetrical with respect to rotation around an axis normal to the glazing.
2. The choice of 45° incidence angle is not directly applicable to all daylighting situations but overall it is more relevant that normal incidence.
3. LDP has been defined as being based on phototropic and visual spectral response (ie. CIE response curve). Standard light meters and halogen illumination will be adequate for spectrically non-selective glazings. Special care should be taken in order to get valid and repeatable results for samples that have strong spectral variations such as radical colors or color dependent scattering materials.
4. The numerical representation of LDP has limited ability to distinguish between glazings which are very poor diffusers and non diffusing glazings. Acid etched glass for example scatters the vast majority of incoming light by only 20º, and therefore almost none would be directed above the horizontal when illuminated at 45º incidence.
Technical Details:
 
Incident radiant specification:
Spectrum : quartz halogenSpatial Uniformity : +/- 5% over an area of 30 cm radius centered on
the sample measurement point.Angular Uniformity: spot source of less than 6″ dia. should be at least 15 ft from sample to provide 2 degrees of angular variation.(The measurements presented were measured with a narrow beam spotlight at a distance of 4.5m and therefore had less than 1º variation.Sample:
Minimum size = 30cm sq
Mask and Aperture:
Mask should be as thin as possible to avoid error at steep angles where the mask would decrease the observed aperture. The aperture should be chosen based on the detector available. It should be narrow relative to the angular sensitivity of the detector.
Detector:
Visible light spectral response. Field of view sufficient to view entire aperture. Limit field of view to minimize stray light entering detector.
Calibration:
Ensure linearity over full sample response. Measure incident light levels with no sample present. Then measure LDP of clear glass. These measurements establish stray light levels and maximize position. Establish signal to noise limits and specify if necessary.