Nyctometry (synonyms: mesoptometry, twilight visual acuity testing) is one of the diagnostic procedures in ophthalmology (eye care) and is used to test mesopic vision or twilight visual acuity, which is mediated by the rods (rods are sensory cells on the retina that are responsible for twilight vision and detect the finest levels of brightness; cones, on the other hand, are responsible for color vision during the day). The term twilight visual acuity is misleading here, since it does not refer to visual acuity in the true sense, but to the complex interaction between visual acuity and contrast sensitivity in the mesopic visual range (twilight vision). The perceptual mesopic contrast threshold determined in nyctometry is referred to as twilight visual acuity. The ability of light-dark adaptation is determined with adaptometry and is to be distinguished from nyctometry
Twilight vision lies between what is known as photopic vision (daytime vision mediated by the cones) and scotopic vision (nighttime vision mediated by the rods). In contrast to daytime vision, visual acuity deteriorates and the threshold for perceiving contrasts increases significantly. This causes considerable difficulties, especially in road traffic. Twilight visual acuity testing to determine nighttime driving ability in driver’s license applicants has not been mandatory since 1999; however, the German Ophthalmological Society continues to recommend that it be performed.
Indications (areas of application)
- The driver’s license fitness examination to test night driving ability.
- The occupational medical fitness examination to test twilight visual acuity and glare sensitivity.
- Additional suspicion of night myopia
- The quantification of visual disturbances at low brightness at:
- Media opacity – e.g., cataract, which is characterized by clouding of the lens of the eye.
- Retinal diseases – e.g. retinopathia pigmentosa, which is associated with progressive nyctalopia (night blindness) due to a degenerative process of the retina
- Pharmacological (drug-related) side effects.
- Neuronal dysfunction – e.g., in the case of damage to the optic nerve (large optic nerve).
The procedure
During the nyctometric examination, the patient is seated in front of the nyctometer, which is located in a moderately lit room. If the subject has been previously exposed to bright light, an adaptation time of approximately 10 minutes should be allowed to avoid falsifying the results. Other light sources in the examination room can be switched off by means of a viewing cup on the device. The German Ophthalmological Society recommends different devices for performing nyctometry:
- Contrastometer (Fa. BKG Medizin Technik).
- Mesoptometer II (Fa. Oculus)
- Nyktometer (Fa. Rodenstock)
The patient now sees through the device visual signs (eg, Landolt rings – rings that are open to one side and are otherwise used for visual acuity testing), which are constant in size and whose contrast can be varied by the examiner. Initially, a high contrast is offered, which is progressively decreased until the mesopic contrast threshold is reached. At each contrast level, three of five visual signs offered must be correctly identified. In addition, nyctometry allows testing of glare sensitivity, or contrast perception in twilight with glare. The complete examination of contrast levels is performed both with and without glare. In this case, the glare is applied by the device from the left, analogous to road traffic, so that conclusions can be drawn about night driving ability. Other glare directions are also possible. In addition, some drivers suffer from so-called night myopia, which can be diagnosed in the course of nyctometry: This is myopia in the dark caused by faulty accommodation. During nyctometry, this possibility is also taken into account and corrected with prescription lenses, so that visual acuity improves instantly. Nyctometry is a useful and recognized procedure that can detect disorders of twilight visual acuity with great accuracy, paving the way for possible further diagnostics to determine an underlying disease.Furthermore, the importance and significance in relation to night driving ability should be emphasized, although the nyctometric examination is no longer mandatory before obtaining a driver’s license.
