How to calculate spread and throw from isocandela diagrams
Photometric isocandle diagrams show what you can think of a s a " sphere of light" with isocandles plotted on its surface. They are contour maps of the intensity of the luminaire. There are many projections of the sphere, you must use the equa l-area (also called equivalent, equiareal or authalic) projection to calculate throw and spread, which are, b y t he way, purely "graphical" calcu lations. To calculate spread: Draw a line from the C=0° G=90° point and make it tangent to the 90% isocandela contour. In the above example this happens at about 17° "East" and 62° "South" The angle at which it hits the edge of the "sphere" is the spread. It is the red diagonal line in the image above, so the spread is 27.2° To calculate throw: The throw is calculated by finding the longitude of the maximum candle intensity, then drawing a "vertical...