Ophthalmology

COLOUR BLINDNESS

180 million people in the world are affected by colour blindness.

Due to this hereditary genetic defect, in its most common form, the person is unable to distinguish red and green with certainty. People suffering from colour blindness perceive different shades of grey.

TEST OF ISHIHARA

The Ishihara test is the most common and quickest method of determining whether a person is colour blind. However, it is also the least accurate.

The Ishihara Test is named after its inventor, Shinobu Ishihara, a professor at the University of Tokyo; consists of 38 coloured plates, referred to as pseudo-isochromatic plates as they consist of dots of different size, with small differences in colouring, saturation and brightness, on which a number or a route in a different colour to the background. The number or route is visible to people with a vision normal colour, but is invisible or hardly recognisable to people with colour blindness.

NAGEL'S ANOMALOSCOPE

The Nagel Anomaloscope is a level II diagnostic tool for the detection of colour vision defects that are relevant to wavelength discrimination.

The instrument consists of a tube with three slits through which yellow, green and red light passes, projecting onto a circular field divided into two halves; the upper half receives the red and green light, the lower half the yellow light.

Nagel's anomaloscope provides an accurate assessment of colour sensitivity:

• the patient uses manual controls to balance red, green and yellow light, varying their intensity in an attempt to match their hue and brightness to the colour of the two half-fields;
• the patient uses specific buttons to define the right colour balance of two half-fields, in various consecutive sequences proposed by the operator.

In any case, the Specialist may always integrate the tests performed with the Anomaloscope, adding further investigations (e.g. Ishihara tables), to supplement and complete the final evaluation.