White Balance Meter is a simple color temperature meter designed to measure a gray card to calculate the color temperature of the ambient lighting on a shoot. Tools like this are often used to help blend their portable lighting with ambient lighting using gels, by determining the approximate color temperature of the room. While there are some color meters on the market for > $1,000, most people only need a simple solution. Obviously, given the advanced light sensing hardware in these hand-held units, you'll get far more accurate readings from them, however White Balance Meter provides a pretty good approximation, suitable for assisting in gel selection.

Use this tool with an 18% gray card, although some users report better results with a white card. Shoot a photo of your card, being careful not to expose the card to the glare of direct light. Once taken, tap on the most consistent area of the gray card and the application will approximate the color temperature based on your selection. Depending on the brightness level of your selection, the color temperature may change slightly as you tap around the card. In the event that WB meter cannot determine the white balance, the output will read, "Out of Range"; try tapping on a different portion or take a second photo.

Additionally, you can shoot a photo through your gels at varying distances to get an idea of how each gel will change the color temperature of the shot.

White Balance Meter is not a finely calibrated piece of optical equipment, like the very expensive color meters on the market; it uses the iPhone camera and does the best it can with general pixel data. What it is, however, is a reasonably good approximation of color temperature that is good enough to work with while blending flash gels.