Lighting
Light Beam Diameter Calculator
Calculate ideal beam diameter from throw distance and full beam angle. The default values illustrate the workflow; replace them with measurements or ratings from one documented condition.
Run the Light Beam Diameter scenario
Use measurements from one operating condition.
Beam diameter: calculation method
This page evaluates diameter = 2d × tan(beam angle ÷ 2). The entered quantities are Throw distance and Full beam angle.
The initial scenario returns 2.68 m. Re-enter field data before using the answer in a design or comparison.
Calculate ideal beam diameter from throw distance and full beam angle. A separate luminous intensity result is available from Candela Lumens Calculator.
Before entering values
Use photometric data for the actual fixture, optic, mounting height, and task plane. Do not combine a worst-case value with unrelated nominal data.
At least one field has a positive lower bound because it appears in a denominator. Check every prefix before entry; micro, milli, kilo, and mega are not interchangeable. If you also need shunt resistance, continue with Shunt Resistor Calculator.
- Throw distance
- Default example: 5 m. Enter throw distance in m.
- Full beam angle
- Default example: 30 °. Enter full beam angle in °.
Interpreting the answer
The primary answer, Beam diameter, describes only the entered scenario. Compare it with initial, maintained, and worst-case lighting conditions.
Store the measurement state and conversion notes with the answer. For the companion illuminance calculation, open Inverse Square Lighting Calculator.
A second scenario
Change Throw distance from 5 m to 6 m with every other entry unchanged. The result changes from 2.68 m to 3.22 m.
The comparison demonstrates sensitivity, not a guaranteed field response.
Checks before using the answer
Manufacturer beam-angle definitions and nonuniform beam edges can change the usable diameter.
The arithmetic intentionally leaves out daylight variation, control behavior, depreciation, and spatial uniformity. Test an additional scenario when an omitted effect has a plausible range.
Record reflectance, maintenance condition, beam definition, and measurement location.
A practical checking sequence
Begin by confirming throw distance at the operating point represented by the other entries. Record whether it came from a meter, nameplate, data sheet, or design assumption.
After calculating beam diameter, compare its magnitude with illuminance, uniformity, driver loading, voltage loss, energy use, and maintained output. An independent order-of-magnitude check can reveal a misplaced decimal or unit prefix.
Keep a baseline and vary the least certain measurement first. A separate analysis may be needed for daylight variation, control behavior, depreciation, and spatial uniformity.
When the inputs are uncertain
The calculator rejects values outside its stated field limits, but a numerically valid entry can still describe the wrong condition. Confirm throw distance and full beam angle against the original source before saving the result.
Use an additional case for driver efficiency, optical loss, surface reflectance, and beam-edge definition when that behavior is not negligible. Keep the baseline intact so the effect of each changed assumption remains visible.