Buying lamps is the end point of lighting design, not the starting point. The beam angle and spacing are where most of the project's value is created or lost, and both of these things can be determined on paper before an official order is placed.

The beam angle is not the full diffusion range of light. It is the angle between the two directions where the light intensity drops to 50% of the peak. Light continues to extend at a lower intensity beyond the boundary, which is why the actual performance of two lamps marked at 60° may be significantly different, and why just looking at this angle will never be enough to meet the following specifications.

The spacing is expressed as a ratio to the installation height, and this ratio is determined by the light distribution. Narrow light distribution can support a larger spacing ratio with the same uniformity requirements because it concentrates light more and misses less to both sides. Once the spacing ratio is pushed to the point where the light distribution cannot support it, typical "scallop patterns" will appear on the scene: a bright band under each row of lights and a dark band between the rows of lights.

The file that determines all this is the light distribution file-the IES or LDT file generated from actual measurement for the specific model. Designers can use it to run a DIALux or Relux simulation, and they can hand over isoilluminance maps, uniformity and maintenance illuminance; these three things together are the only convincing evidence that "this cloth lamp solution can meet the standard." If the supplier cannot produce the light distribution documents for the reported model, it is equivalent to requiring the buyer to accept a layout in an unverifiable way.