The five surface-brightness versus color-index relations used in SearchCal for faint objects to determine the limb-darkened angular radius of calibrators. From left to right and top to bottom we show ψ_I(I − J), ψ_I(I − K), ψ_J(J − H), ψ_J(J − K), ψ_H(H − K). The red line is our polynomial fit. Symbols for different luminosity classes: asterisk (super-giants I and bright giants II), open triangle (giants III), filled square (sub-giants IV), filled pentagon (dwarfs V). Data from direct angular diameter measurements are plotted in black and data from eclipsing binaries in red.