Understand the physical quantity
What is Point-Source Illuminance and Distance?
Photometry describes light as weighted for human vision, while radiometry describes physical radiant power. Flux, intensity, illuminance, luminance, irradiance, wavelength, frequency, and photon energy are related but distinct quantities.
Combine distance and receiving-surface orientation in one reversible photometric check rather than forcing visitors through separate calculations.
The relationship
Keep quantity and basis explicit
For a point source, illuminance E = I cos(θ)/d², where I is luminous intensity toward the point, d is distance, and θ is the angle from the receiving surface normal. Each solve mode uses this same relationship.
See the workflow
What the calculator is doing
1Source or measured light2Apply geometry or spectrum3Photometric or radiant result
Worked context
Read the output in context
At 2 m, 1,000 cd arriving 30° from the surface normal produces about 216.506 lux.
Interpret with care
Important boundary
The point-source law is unsuitable close to an extended source and does not account for obstruction, reflections, beam nonuniformity, surface shape, spectral response, measurement uncertainty, or multiple sources.
The result does not select a lighting standard, certify compliance, predict perception, or replace calibrated photometry and optical design.
Browse Light & radiation for related physical quantities.
Quick guide
How to use this calculator
- Choose the photometric, radiometric, or photon relationship first, then enter measurements using the stated geometry and unit basis.
- Keep luminous and radiant quantities separate. The calculator performs only the declared conversion, reverse solve, geometry, or measurement summary and exposes useful intermediate values.
- Use companion outputs to reconcile the scenario, then retain the stated instrument, distribution, spectrum, geometry, and environmental boundaries before applying the result.
Calculation method
How the point-source illuminance and distance calculator works
Combine distance and receiving-surface orientation in one reversible photometric check rather than forcing visitors through separate calculations.
For a point source, illuminance E = I cos(θ)/d², where I is luminous intensity toward the point, d is distance, and θ is the angle from the receiving surface normal. Each solve mode uses this same relationship.
Worked example
Point-Source Illuminance and Distance example
At 2 m, 1,000 cd arriving 30° from the surface normal produces about 216.506 lux.
For a point source, illuminance E = I cos(θ)/d², where I is luminous intensity toward the point, d is distance, and θ is the angle from the receiving surface normal. Each solve mode uses this same relationship.
Supported inputs
Precision and limits
Light and optics boundary
The point-source law is unsuitable close to an extended source and does not account for obstruction, reflections, beam nonuniformity, surface shape, spectral response, measurement uncertainty, or multiple sources.
Units and precision
Calculations normalize compatible units once, retain working precision, and round only for display. Very small and large nonzero values use scientific notation; displayed precision cannot exceed the entered measurements or source data.
Decision boundary
This calculator does not select a lighting standard, certify compliance, predict perception, or replace calibrated measurement, photometric distribution data, optical design, or safety assessment.
Privacy
Entered values and results stay in this browser and are not sent to analytics or third parties.
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