A simple experimental method for evaluating small birefringences is reported. This method is based on the amplification of low phase retardation values using a rotary (lambda) plate in conjunction with the birefringent sample. The new value of retardation is readily evaluated in white light with the sensitive (lambda) plate or is measured with a system of a compensating plate and wedge in monochromatic light. The initial small retardation is determined by interpolation from theoretical curves. This unexpensive method can be applied in any biological, chemical or mineralogical laboratory where simple polarizing accessories are available.