Beam intensity in candela from a field reading, or from photometric test data at a given aiming angle.
Two routes to the same quantity. Convert an illuminance reading taken at a known distance into candela, and read the intensity your fixture's photometric data gives in a chosen direction once the aiming angles are applied. The aiming table then shows how much a small change of toe-in or elevation would move that value.
Angle between the sensor normal and the line to the fixture. 0 = sensor square-on.
Used only to check the distance is long enough for the inverse-square law.
Top row: horizontal angles. First column: vertical angles. Paste from a spreadsheet or use commas. Angles are measured from the fixture's own beam axis.
Direction from the fixture to the observer, in site angles. Use one sign convention for aim, observer and grid. Vertical is positive upward.
Take the minimum from your governing standard or the project specification. 0 skips the check.
Intensity toward the observer if the fixture were re-aimed. Rows are elevation, columns are toe-in. The highest value is highlighted.
Every step is shown so the result can be checked by hand.
Illuminance falls with the square of distance. If the sensor is tilted, divide by the cosine of the tilt to get the value on a plane facing the fixture.
Lux and metres give candela directly. Foot-candles and feet also give candela, since 1 fc × 1 ft² = 1 cd.
The observer direction is turned into the fixture's own frame by undoing the toe-in and then the elevation. This is exact, not a simple subtraction, so it stays correct for larger angles.
Intensity at (H′, V′) is interpolated linearly between the four surrounding grid points. A finer grid gives a better answer, particularly near the peak. Directions outside the grid report no value rather than zero.
| Item | Value |
|---|---|
| 12.5 lux at 20 m, sensor square-on | 5,000 cd |
| Grid, aim 0° / 0°, observer 2.5° / 1.5° | 5,000 cd |
| Interpolation | (10000 + 4000 + 4000 + 2000) ÷ 4 |
| Share of 10,000 cd peak | 50 % |
| Predicted illuminance at 20 m | 12.5 lux |
| Aim 1° toe-in and 1° up | about 7,400 cd |
It does not model atmospheric loss, fixture roll, colour filters, sensor spectral error or flashing (effective) intensity. It does not average intensity over a beam pattern, and it holds no minimum-intensity values, so you enter the requirement from the standard you work to. The grid in the example is illustrative and must be replaced with certified data.
Quick answers on converting field readings to candela and reading intensity from photometric data.
It finds beam intensity in candela in two ways. One converts an illuminance reading taken at a known distance into candela. The other reads the intensity your fixture's photometric data gives in a chosen direction once the aiming angles are applied. An aiming table then shows how much a small change of toe-in or elevation would move that value.
Illuminance falls with the square of distance, so En = E ÷ cos φ and I = En × d², where φ is the sensor tilt and d the distance. Lux and metres give candela directly, and foot-candles and feet also give candela, since 1 fc × 1 ft² = 1 cd. For example, 12.5 lux at 20 m with the sensor square-on gives 5,000 cd.
The observer direction is turned into the fixture's own frame by undoing the toe-in and then the elevation, which is exact rather than a simple subtraction. The intensity at (H′, V′) is interpolated linearly between the four surrounding grid points, and directions outside the grid report no value rather than zero. A finer grid gives a better answer, particularly near the peak.
No. The grid shown is illustrative, not manufacturer data. Replace it with certified photometric data. Angles are measured from the fixture's own beam axis, and you should use one sign convention for the aim, the observer and the grid, with vertical positive upward.
It shows the intensity toward the observer if the fixture were re-aimed, with rows for elevation and columns for toe-in, and the highest value highlighted. In the worked example, re-aiming by 1° toe-in and 1° up gives about 7,400 cd. The step size of the table is set by you.
The fixture's largest dimension is optional and is used only to check that the distance is long enough for the inverse-square law to hold. The sensor incidence angle is the angle between the sensor normal and the line to the fixture, where 0 means square-on, and a tilted sensor is corrected by dividing by the cosine of the tilt.
Yes, if you enter a required minimum in candela, but it holds no minimum-intensity values itself. Take the minimum from your governing standard or the project specification. A value of 0 skips the check. Compliance shall be verified against the applicable standard and the manufacturer's certified photometric data.
It does not model atmospheric loss, fixture roll, colour filters, sensor spectral error or flashing (effective) intensity, and it does not average intensity over a beam pattern. The comparison of the field reading with the data is meaningful only if the reading was taken in the observer direction.
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