Rod photoreceptor function is reduced in many retinal diseases including retinitis pigmentosa [1–3], rod-cone dystrophy , retinal telangiectasia  and congenital stationary night blindness . Recent histological evidence shows that rods may be affected prior to cone photoreceptors in the early stages of age-related macular disease (AMD) and psychophysical data also show a selective impairment of parafoveal rod photoreceptors in AMD . Consistent with this, people with AMD report particular difficulty with vision under dark-adapted conditions .
Currently, rod function is measured in clinical studies by measuring the scotopic electroretinogram [10, 11], by measuring dark adaptation [12, 13], or by performing dark-adapted perimetry . The instrument most currently used for performing dark-adapted perimetry is a modified first-generation Humphrey Field Analyser (HFA, Carl Zeiss Meditec Inc, USA) [8, 14–18]. This instrument relies on technology first developed in the 1980 s  such as 5.25" floppy discs and a 'light-pen' to enter data. More significantly, it does not correct for poor or unstable fixation. Although the second generation HFA does include an infrared eyetracker and is still commercially available, a custom chipset is required to over ride the self-calibration programme . Further, this instrument does not measure the position of gaze, only changes in gaze from the initial calibration position. The instrument can, therefore, identify poor fixation but can not correct for it.
In one conference abstract it has been reported that scotopic perimetry has been performed with a confocal Scanning Laser Ophthalmoscope  but to the best of our knowledge this has not been repeated by others.
The MP-1 microperimeter (Nidek Technologies, Italy), launched in 2002, is a system which performs retinal-specific microperimetry [22, 23]. It comprises an infrared camera which provides a retinal image, updated at 25 Hz, and a LCD display which can be used to present stimuli. It also incorporates a fundus camera to capture a full-colour retinal image, and inbuilt software can superimpose the microperimetry plot onto the retinal image. This software includes several perimetry strategies (including the Humphrey 10-2 strategy), and it is straightforward to program new paradigms by specifying the test locations, target size, target exposure duration, background luminance, thresholding strategy and fixation target.
Under fully dark-adapted conditions, a central scotopic scotoma exists in people with healthy retinas, corresponding to the foveal rod-free region. This rod-free region measures about 0.35 mm and subtends approximately 1.25° . Peak rod density is reached at approximately 5° eccentricity, so a relative dark-adapted scotoma can be found within the central 10° of retina.
Here we discuss modifications which can be made to the Nidek MP-1 microperimeter to enable scotopic microperimetry to be performed with this instrument. We show that the technique is operating under scotopic conditions by measuring the foveal scotopic scotoma in ten subjects. We also compare these data to those collected with the modified HFA.