ABSTRACT
PURPOSE: To perform non-contact applanation tonometry over soft contact lenses and to examine any differences between the measured IOPs with and without the contact lens. METHOD: IOP was measured at one eye of eight subjects and re-evaluated through a soft lens in situ over the cornea. Seventy percent (ES70, Lunelle) water contact lenses or powers ranging from -15.00 D to +13.00 D were used. RESULTS: Mean IOP without lenses was 15.2 mmHg (S.D. = +/-4.2) and this did not change significantly for lenses ranging from -15.00 D to +3.00 D (P > 0.01) but did change for lens powers +6.00 D and above (P<0.01). The least squares line describing the change in measured IOP (deltaIOP) as a function of lens power (x) has the following characteristics, deltaIOP = 0.022 + 0.591x + 0.11x2 + 0.005x3 (r = 0.985, P = 0.0001). CONCLUSION: Non-contact tonometry can be performed with sufficient accuracy over a soft lens on condition: (a) lens centre thickness is no more 0.30 mm and (b) power is not greater than +3D.
ABSTRACT
AIMS: The National Screening Committee (NSC), whilst recommending the use of digital mydriatic retinal photography for diabetic retinopathy screening, has not yet accepted the use of digitally compressed images for grading. By greatly reducing the file size, however, compression of images is invaluable for storage and for its rapid transmission across computer networks. We undertook a study to compare the different levels of JPEG compression with the original bit-mapped image to determine whether there was any loss of clinical detail following compression. METHODS: Three hundred and thirty images were analysed in this study. These images had been captured from 66 eyes consecutively photographed in a diabetic retinopathy screening programme, using a Sony DXC-950 P 3CCD colour video camera mounted on a Canon CR6-45NMf fundus camera. Single 45 degrees macula-centred images were taken from each eye. The images were compressed using the JPEG algorithm within Adobe Photoshop (version 4.0) and then displayed with a Sony Trinitron colour monitor. Four different levels of compression were used, JPEG-1, JPEG-2, JPEG-3, JPEG-4, and an objective analysis was undertaken using 'lesion counts'. The compressed images were assessed separately and blindly and the results compared with their original BMP images. RESULTS: Eight BMP images could not be evaluated (five right eye and three left eye). A total of 290 images were therefore used in the final evaluation. All the JPEG-1 images with file sizes between 16 and 24 kb were found to be 'pixelated', while the JPEG-4 images (66-107 kb) appeared similar to the original BMP (1.3 Mb) images. Both JPEG-2 and JPEG-3 images had significantly lower counted lesions than the BMP images. CONCLUSIONS: From our findings we can conclude that only some degree of image compression (compression ratios of 1 : 20 to 1 : 12) with file sizes of 66-107 kb is permissible using JPEG format, whereas the images obtained after higher compression ratios may not be suitable for diabetic retinopathy screening.
