The optical loss co-efficient in InGaN laser diodes, emitting at similar to 410nm, has been measured. The measurement technique is based on the transmission of internally generated spontaneous emission through varying lengths of the laser waveguide. It is unique in that it provides spectral and spatial information on the optical loss. The lasers studied re typical of InGaN structures showing a high degree of waveguide loss, alpha (i) = 40 cm(-1). The measurements also show clear evidence of higher order transverse modes in the direction perpendicular to the growth plane with resonant leakage of the optical field into the outer layers of the structure. This produces a modulation in the loss of these modes.