Table 2.

Rod ‘absolute’ threshold, i.e. the threshold number of photons at the cornea when using the most lenient response criterion (middle column). Sakitt's (1972) average rod threshold for three subjects is shown for comparison. Values reported for subjects 1 and 4 are the mean and range for two data sets. To facilitate comparison across subjects the criterion-free 2AFC thresholds predicted from each subject's rating scale data using the method described in Sakitt (1972) are also shown (right column). Note the agreement in both data sets.

	Rod Threshold at lowest criterion (photons at the cornea)	Predicted 2AFC Thresholds (photons at the cornea)
Subject 1	44 (27 - 62)	47 (23 - 71)
Subject 2	70	74
Subject 4	54 (46 - 62)	55 (51 - 59)
Subject 6	32	32
Average	50 ± 16	52 ± 17
Sakitt	42 ± 15	50 ± 8