the &# 34 ; once - around &# 34 ; stepwise density drop is most obvious if the roller surface material has a heat capacitance and a heat transfer coefficient near that of the media . other disclosures have described using roller materials with a very high or a very low heat transfer coefficients to reduce the visibility of this artifact . a very large heat transfer coefficient causes the tread and riser of the &# 34 ; once - around &# 34 ; density steps to tilt and become a density ramp . this is because the internal heat transfer in the roller becomes great enough to maintain the surface temperature of the roller near the average temperature of the roller as it loses heat to the film . a very low heat transfer coefficient in the roller material reduces the height of the density steps by reducing media heating rate differences causes by the changing roller temperatures . reducing the &# 34 ; once - around &# 34 ; density step visibility by reducing the heat transfer coefficient of the roller has the advantage of improving in - sheet and sheet - to - sheet processing uniformity compared to using the high heat transfer coefficient approach . the main difficulty in taking this approach is finding a durable roller material with a low enough heat conductance and / or heat capacitance to make the &# 34 ; once - around &# 34 ; density steps so small in magnitude they cannot be seen . fig1 represents a plot of density along the length of a flat field exposure which shows three lead edge &# 34 ; once - around &# 34 ; steps . such a result implies that the first few rollers which contacted the film contributed heat to warm the film during their first three rotations . with each rotation of the rollers against the film the rollers cooled and had less energy to transfer to the film the following rotation . by the time the rollers made three rotations they were near their equilibrium temperature with the film and their continued temperature change was too small to visibly affect film density . the number of rollers which contribute to lead edge &# 34 ; once - around &# 34 ; density steps depends on the media velocity , the diameter and spacing of the pressure rollers , the heat transfer and heat capacitance characteristics of the drum , pressure roller and media materials . if , for example , the film has reached the drum temperature by the time it reaches the third pressure roller , only the first two rollers can contribute significantly to the phenomena since the third roller is not cooled by the film . if doubling the film velocity result in the film reaching the drum temperature by the time it reaches roller six , the first five rollers contribute to the phenomena . in a real system , of course , the film never fully reaches the drum temperature and therefore even the last roller adds a tiny contribution to the artifact . it should be noted that the number of &# 34 ; once - around &# 34 ; steps visible on the film is generally not the same as the number of rollers which significantly contribute to the steps . with our 0 . 5 &# 34 ; diameter silicone foam pressure rollers and 0 . 030 &# 34 ; thick 1 w / m ° k high conductivity silicone on a 6 &# 34 ; diameter thermal drum with 180 ° film wrap , the first once around is generally the only one visible with the second &# 34 ; once - around &# 34 ; only just noticeable on the most sensitive media processed at the high end of processing temperature range . the density drop ( δd ) at the end of the lead step we can often see with silicone foam rollers is generally 0 . 02 or less . a simulation of the visual impact of breaking the 0 . 02 δd edge at the end of the first &# 34 ; once - around &# 34 ; step into offset substeps was performed and it confirmed the potential effectiveness of the approach . for this simulation , the assumption was made the contribution of the n rollers most involved was equal and that the 0 . 02δd density step could be divided into n substeps of 0 . 02 / n δd . by the time the &# 34 ; once - around &# 34 ; step was divided into four substeps ( case iii ) the artifact printed on the hene film with the 2180 printer was barely visible . this result is due tot he fact that the density difference between two neighboring substeps ( 0 . 005 ) is smaller than the threshold value of discernible density difference by the eye which is 0 . 0052 . in these simulations , the step edge was much sharper than is seen with the foam rollers which yield a fuzzier edge . consequently , dividing the 0 . 02 δd density step into two or three substeps in a foam roller processor would probably suffice . tests were also run on a foam roller processor breadboard to evaluate the practicality of this approach . roller configurations were tested with : all 0 . 5 &# 34 ; diameter rollers ; one 0 . 375 &# 34 ; roller in position one and the rest 0 . 5 &# 34 ;; and with two 0 . 375 &# 34 ; rollers followed by 0 . 5 &# 34 ; rollers . the results showed a progressive reduction of the visibility of the step . even with two leading 0 . 375 &# 34 ; diameter rollers , the π * 0 . 375 &# 34 ; step edge was not yet discernible , indicating many rollers are involved in this artifact process . in this 14 roller breadboard , four to six leading 0 . 375 &# 34 ; diameter rollers would be needed to break the step into two equal barely discernible substeps . a surefire way of evenly dividing the step density is to alternate roller diameters throughout the processor . this yields a design in which equal density division is certain and is insensitive to material property or process time variations . hot drum processors which hold the film in contact with the drum by means of one or more contact rollers have the potential problem of printing out a lead edge dark hand π × d r long where d r is the holddown roller diameter . in some cases , depending heat capacitance and conductance of the roller , two or more decreasing density steps are printed , each πd r long to give a density profile on a uniform exposure sheet of media as shown in fig1 . this artifact can be eliminated by having a roller which has a heat transfer coefficient of 0 . the staircase density drop can be changed to a product ramp which cannot be detected by the eye by using highly internally conductive rollers which have a conductivity approaching ∞. this insures the surface temperature is always equal to the average temperature of the roller , resulting in a gradual density drop until the roller reaches a steady state temperature . this generally requires several sheets of film fed head - to - tail to achieve steady state . an asymptotic density drop along the length films required to reach steady state can be measured . these in - sheet or sheet - to - sheet density drops can be eliminated or reduced by resheeting the entrance roller ( s ) as previously described . when using low density silicone foam rollers , the thermal conductivity is in the 0 . 06 w / m ° k range and the visible effect on the lead edge is a 0 . 01 to 0 . 03 lead edge once around π * d r long . generally , the second once around step is too slight to be visible . replacing the first roller with a solid stainless steel roller only slightly reduces the magnitude of the step indicating the first two to four rollers are involved in printing this artifact . ( an all stainless steel roller processor can produce a lead edge once around step in the 0 . 001 - 0 . 003 range .) how readily a 0 . 02 density step can be seen as a function of how sharp the transition is for a given viewing distance . to reduce the visibility of the artifact , i propose using graduated diameter nip rollers starting with a 0 . 375 &# 34 ; diameter first roller . ( smaller would be better , but for a 144 &# 34 ; span , a practical limit is around 3 / 4 &# 39 ; for stiffed reasons .) this yields a 1 . 18 &# 34 ; long step . if three rollers are involved for a given foam in producing the artifact and the final roller diameter is to be 0 . 540 &# 34 ; which yields a 1 . 7 &# 34 ; long step , the second roller should be 0 . 46 &# 34 ; diameter to provide an intermediate length once around contribution 1 . 44 &# 34 ; long . this is shown in fig2 a . this makes a minor artifact almost invisible to the eye . in this case , the fifth through the last roller could revert back to ≈ 1 / 2 &# 34 ; rollers which can be closely spaced to suppress the lead edge and trail edge density droop which is proportional to the nip to nip spacing . this is shown in fig4 . the lead and trail edge density droop is shown in fig5 . it is caused by the film ends separating from the drum as they travel the distance between rollers . reducing the nip to nip distance or increasing the drum diameter reduces the lead and trail edge density droop . the invention has been described in detail with particular reference to certain preferred embodiments thereof , but it will be understood that variations and modifications can be effected within the spirit and scope of the invention .