A binocular pupillometer apparatus for assessing relative afferent pupillary defect (RAPD) in a subject comprising: a binocular pupillometer comprising stimulating means arranged to apply a plurality of stimulating visible light pulses of different intensities independently to each eye and sensing means arranged to generate and record separate pupillary response data in respect of the left and right eyes, the apparatus further including processing means arranged to calculate RAPD of a subject by: (i) determining an apparent pupillary response for each of the left and right eyes with respect to intensity from the recorded pupillary response data, wherein the apparent pupillary responses of the left and right eyes are not parallel to each other; (ii) calculating an area difference D between the apparent pupillary responses of the left and right eyes with respect to intensity; characterised in that the processing means is arranged to calculate RAPD of a subject by further either: (iii) generating modified pupillary responses for the left and right eyes which are parallel to each other, have a gradient which is intermediate the apparent pupillary responses of the left and right eyes and are separated by the area difference D; (iv) calculating a value for RAPD based on the modified pupillary responses of the left and right eyes; and (v) outputting said value for RAPD which is based on the modified pupillary responses, wherein the processing means is arranged to calculate the gradient of the modified pupillary responses by determining an average of the gradients of the apparent left and right eye pupillary responses, and the processing means is arranged to calculate the area difference D between the apparent pupillary responses by integrating the amplitude of each of the apparent pupillary responses with respect to intensity and subtracting one from the other, or: (iii) calculating an intermediary pupillary response with respect to intensity which is intermediate the apparent pupillary responses of the left and right eyes and determining a modified pupillary response for the left or right eye with respect to intensity which follows the intermediary pupillary response and is separated from the modified pupillary response of the left or right eye by an area difference D/2; (iv) calculating a value for RAPD based on the intermediary pupillary response and the modified pupillary response of the left or right eye; and (v) outputting said value for the RAPD which is based on the intermediary and the modified pupillary response, wherein the processing means is arranged to calculate the gradient of the intermediary pupillary response and the modified pupillary response by determining an average of the gradients of the apparent left and right eye pupillary responses, and the processing means is arranged to calculate the area difference D between the apparent pupillary responses by integrating the amplitude of each of the apparent pupillary responses with respect to intensity and subtracting one from the other. (i) determining an apparent pupillary response for each of the left and right eyes with respect to intensity from the recorded pupillary response data, wherein the apparent pupillary responses of the left and right eyes are not parallel to each other; (ii) calculating an area difference D between the apparent pupillary responses of the left and right eyes with respect to intensity; characterised in that the processing means is arranged to calculate RAPD of a subject by further either: (iii) generating modified pupillary responses for the left and right eyes which are parallel to each other, have a gradient which is intermediate the apparent pupillary responses of the left and right eyes and are separated by the area difference D; (iv) calculating a value for RAPD based on the modified pupillary responses of the left and right eyes; and (v) outputting said value for RAPD which is based on the modified pupillary responses, wherein the processing means is arranged to calculate the gradient of the modified pupillary responses by determining an average of the gradients of the apparent left and right eye pupillary responses, and the processing means is arranged to calculate the area difference D between the apparent pupillary responses by integrating the amplitude of each of the apparent pupillary responses with respect to intensity and subtracting one from the other, or: (iii) generating modified pupillary responses for the left and right eyes which are parallel to each other, have a gradient which is intermediate the apparent pupillary responses of the left and right eyes and are separated by the area difference D; (iv) calculating a value for RAPD based on the modified pupillary responses of the left and right eyes; and (v) outputting said value for RAPD which is based on the modified pupillary responses, (iii) calculating an intermediary pupillary response with respect to intensity which is intermediate the apparent pupillary responses of the left and right eyes and determining a modified pupillary response for the left or right eye with respect to intensity which follows the intermediary pupillary response and is separated from the modified pupillary response of the left or right eye by an area difference D/2; (iv) calculating a value for RAPD based on the intermediary pupillary response and the modified pupillary response of the left or right eye; and (v) outputting said value for the RAPD which is based on the intermediary and the modified pupillary response,