in the pictorial field in fact variations in light output of up to 2 1 have been experienced and such variations are noticed even in two dimensional arrays many facsimile receivers however employ a linear one dimensional array rather than a two dimensional array with a number of led in the linear array equal to the number of pels in one scan the pictorial field is developed in such receivers by moving a suitable print medium across the array as successive scans are illuminated in such receivers even very slight differences in leds light outputs are noticeable these differences manifest themselves as longitudinal striations across the pictorial field commonly referred to as artifacts linear arrays are light sensitive in still another way two dimensional led arrays produce satisfactory multitone pictorial fields even with a low number of bits in the data words inasmuch as changes in gray level are expected from pel to pel and from scan to scan no artifacts show up operating two dimensional arrays with only few bits to define the gray levels is therefore quite feasible with a linear array on the other hand the accuracy of each level must be tightly controlled to prevent artifacts even if the number of gray levels is relatively small therefore the number of bits employed to effectively define each gray level in a led facsimile system having a linear array is larger than the number of bits required to distinguish a desired number of gray levels for example with 15 gray levels characterizable by five bits it has been found that the use of seven bit words to accurately.