# What filter do I need to convert a xK lamp to a colour temperature of yK?

For example converting a 4200k lamp to 6000K you need to do the following calculation. Divide initial source into 1,000,000 = 1,000,000 / 4200 = 238 mireds, then divide the desired colour temperature into 1,000,000 = 1,000,000 / 6000 = 167 mireds. Subtract sum one from sum two 167 – 238 = -71 mired shift (the minus sign is important!). Select the filter with the nearest mired shift to -71, in this case it is 202 Half CT Blue (-78 mired shift). For mired shift values see our mired shift calculator on this website.