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

School of Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK

Correspondence to:
Correspondence to:
James S Wolffsohn
School of Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK; j.s.w.wolffsohn{at}aston.ac.uk

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 (2048x1360 pixels), DVC:1312C (1280x811), and JAI:CV-S3200 (767x569) single chip cameras and the JVC:KYF58 (767x569) 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 1280x1024) 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 25x magnification. Image quality was perceived subjectively as being reduced when the pixel resolution was lower than 767x569 (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 1280x811 and 767x569 pixel resolution and at up to 1:70 JPEG compression.

Abbreviations: BMP, bit mapped graphics format; JPEG, Joint Photographic Experts Group; SVGA, super video graphics array; TIFF, tagged image file format

Keywords: resolution; compression; anterior eye imaging; digital grading