Simply hold OIO Loop at a 45° angle while directing it at OIO bulb. You should see a rainbow stretched along the length of the lens. For lights in the daytime, you’ll want to see bright blue close to the green area. For lights in the evening, there shouldn’t be any blue near the green, but OIO Bulb will show violet after a large gap with the green area. This violet light can appear more blue than you expect. The placement of the light in the rainbow will show you the approximate wavelength, so a gap will tell you this is violet.