Bubble sizing by optical means is very common, but requires calibration by non-optical means. This is particularly important since apparent bubble size increases with decreasing threshold intensity. A calibration experiment was conducted comparing the displaced water mass from captured bubbles with backlit optical bubble size. The results showed that the appropriate threshold is slightly below the local background intensity. The cause was from off-axis light from the background illumination screen forward reflecting off the bubble edge into the camera. This reflection causes a bright ring which obscures the true bubble edge. An equation describing the forward reflection obscuration was derived and was in agreement with the bias in the optical bubble size. Neglecting this effect, particularly if an inappropriate threshold is used, may cause bubble size underprediction as large as 10% to 15%. The results suggest thresholding at multiple intensities and using a curve fit to extrapolate to the appropriate size will produce the greatest accuracy. © 2003 Elsevier Ltd. All rights reserved.