# Q&A: Understanding Candela Plot Orientation (this KB article contributed by a TracePro customer)

Synopsis
Understanding Candela Plot Orientation (this KB article contributed by a TracePro customer)

Solution
Candela Plot Orientation
The candela settings require you to set up two vectors that define the candela coordinate system with respect to the TracePro global coordinate system. The Candela coordinate system vectors are:
“Normal” – which defines the Z-axis of the candela coordinate system.
“Up” – which defines the Y-axis of the candela coordinate system.
X-axis, which is determined by the right-hand rule.

How Candela Sweeps through the axes
The lines on the candela plot are created by sampling the tip of the Z vector as it rotates about the X-axis. Candela sweeps are limited to +-90 degrees. Use the right-hand rule (RHR) about the X-axis to determine sign of the angle.

Choose a horizontal angle: The original Y-axis direction is “zero” horizontal angle (the coord system starts at zero horizontal). To change the horizontal angle, rotate the Y vector about the Z-axis between 0-180 degrees. Using RHR about the Z-axis to make sure you are rotating in the positive angle direction.

Limited Candela Capability
The candela coordinate system only traces out a hemisphere because the candela sweep is limited to +-90 degrees about the Z axis. Horizontal angles go from 0->180 degrees, but again that only gets you a hemisphere.

The elevation sweep done during a candela scan only samples light that contacts the Z vector “bin” as it sweeps. That is why there are other horizontal axes. The light will be distributed on other horizontal bins.

30051 新竹市經國路二段160號9F-8
Tel : 03-5359299  Fax: 03-5358908