Light Diffusing Power (LDP) is a new measurement and rating system that allows building designers to objectively compare light diffusing ability of various glazings. 

Direct sun strikes glazing at 45° and enters an idealized space 
Radiance Renderings: 

Graphical LDP: 

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 0.001.00, and a Descriptive rating [nondiffusing, 

Objectives:
LDP was designed to meet the following objectives: 

Setup:  
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: 


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 090° 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


401401

0.30


300300

0.73

Good


300545

0.91

Excellent


545545

0.95

Limitations and Comments: 
Technical Details: 
Incident radiant specification: Spatial Uniformity : +/ 5% over an area of 30 cm radius centered on 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. 