Read iso-candela and iso-illuminance plots
Understand contour plots that show how light spreads horizontally — both as candela (raw output) and illuminance (footcandles on the ground).
4 steps · 3 min read
Iso plots translate the polar plot's vertical-angle data into the horizontal map you actually have to deliver lighting on. Reading them well lets you spot beam overlaps, dark spots, and spill before you ever lay out a fixture.
Find the iso plots
Scroll past the polar / linear charts on the Photometric tab. You'll see two iso plots: Iso-candela (a top-down map of candela in the lateral plane) and Iso-illuminance (footcandles on a notional ground plane at the fixture's mounting height).
How to read the contours
Each closed contour line is a constant candela (or footcandle) value. Where contours bunch up tightly, light is changing rapidly — that's typically the edge of the beam. Wide spacing means a smooth, even gradient.
The legend on the side of the plot tells you which color matches which value.
Compare iso-candela vs iso-illuminance
Iso-candela is a property of the fixture alone: it's the same regardless of mounting height. Use it to compare two fixtures' beam shapes.
Iso-illuminance is the same data projected onto a flat surface at the fixture's mounting height. The values change as you change the mounting height, and the contours stretch and compress as the geometry changes.
For real-world layout decisions you almost always care about iso-illuminance, but iso-candela is the right tool when you're comparing pure fixture performance.
Mounting height matters
Set the mounting height at the top of the iso-illuminance card to see how the footcandle pattern changes. Doubling the mounting height roughly halves the peak illuminance and broadens the spread (inverse-square law).
Keep an eye on whether the contours you care about (1 fc, 5 fc, etc.) still cover your target area.