Peter A. Hardy PhD, Presenter: Nothing to Disclose

Medical Image monitors used for image interpretation on MR and CT systems applying for ACR accreditation must be assessed for adequate luminance range and uniformity. Typically this is done with a calibrated spot luminance meter. These devices are expensive and are not readily available to most radiology departments. We sought to calibrate a digital camera so it could be used to estimate the luminance of medical displays.

We used three digital cameras (Canon Power Shot SX100, Canon PowerShot SX10 and a Nikon D60) to take photographs of a flat panel LCD monitor displaying a SMPTE pattern. The luminance of the varying contrast squares of the SMPTE pattern were measured using a calibrated photometer.  The photographs were exported from the cameras as JPEG files and analyzed in a program developed in IDL to measure the average grey value within ROI encompassing 75% of the area of each contrast block. We fitted the grey values to a polynomial equation to derive an equation L=f(G) linking the photographic grey values (G) to the monitor luminance (L). We then tested this equation by taking similar photographs of a second monitor and comparing the luminance estimated from the photograph against that measured using the photometer.

The relationship between monitor luminance and photographic grey value was best parametrized with a quadratic function where the grey value was transformed to x= log(1-G/Gmax) where Gmax was 255 as the photographs were three byte (RGB) JPEG files. Each camera had a unique, but similar, opto-electronic conversion function (OECF). Using the OECF we confirmed the luminance could be corrected for different exposure settings (EV) on the camera, (ISO, F-stop, shutter speed).This allowed us to estimate luminance even if the EV were different between calibration and use. The ratio of the estimated to the measured luminance was 1 to within a small value (10%, 5%, 3% for the three cameras) over the luminance range 0 - 90 Cd/m2 .

With careful calibration a digital camera can measure monitor luminance rapidly and repeatedly. Photographs can be stored for subsequent review. It obviates the need to acquire an expensive and seldome used piece of equipment.

Hardy, P, The Use of a Digital Camera to Measure the Luminance of a Medical Monitor.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13028316.html