The effect of digital image resolution and compression on anterior eye imaging

Abstract

Aim: To determine the theoretical and clinical minimum image pixel resolution and maximum compression appropriate for anterior eye image storage.

Methods: Clinical images of the bulbar conjunctiva, palpebral conjunctiva, and corneal staining were taken at the maximum resolution of Nikon:CoolPix990 (2048×1360 pixels), DVC:1312C (1280×811), and JAI:CV-S3200 (767×569) single chip cameras and the JVC:KYF58 (767×569) three chip camera. The images were stored in TIFF format and further copies created with reduced resolution or compressed. The images were then ranked for clarity on a 15 inch monitor (resolution 1280×1024) by 20 optometrists and analysed by objective image analysis grading. Theoretical calculation of the resolution necessary to detect the smallest objects of clinical interest was also conducted.

Results: Theoretical calculation suggested that the minimum resolution should be ≥579 horizontal pixels at 25× magnification. Image quality was perceived subjectively as being reduced when the pixel resolution was lower than 767×569 (p<0.005) or the image was compressed as a BMP or <50% quality JPEG (p<0.005). Objective image analysis techniques were less susceptible to changes in image quality, particularly when using colour extraction techniques.

Conclusion: It is appropriate to store anterior eye images at between 1280×811 and 767×569 pixel resolution and at up to 1:70 JPEG compression.