with reference to fig1 there is shown a cross - sectional view of an optical recording medium which includes a disc substrate 1 , which may be a resin substrate formed from poly - carbonate or other similar material . the disc substrate 1 may have grooves preformed therein for guiding the laser light , shown as incident on the disc in the direction of arrow 8 . alternatively , disc substrate 1 may be a glass plate formed by the 2p process , a substrate prepared by directly forming grooves on a glass plate , or a substrate having bit rows for guiding laser light preformed therein . as shown in fig1 a first dielectric layer 2 , approximately 160 nm in thickness , which may consist of a mixed film of zns and sio 2 , is formed on top of disc substrate 1 . the first dielectric layer 2 has deposited thereon a thin film recording layer 3 having a thickness of approximately 20 nm . thin film recording layer 3 may have a composition whose main component is te - ge - sb . a second dielectric layer 4 covers thin film recording layer 3 , and may be formed from the same composition as the first dielectric layer 2 . however , the second dielectric layer 4 is generally much thinner than the first dielectric layer 2 , and optimally has a thickness of approximately 20 nm . covering the top of the second dielectric layer 4 is a reflecting layer 5 , which may be an aluminum alloy . finally , to complete the optical recording medium , a protective plate 7 is adhered to the top of the reflecting layer 5 by means of an adhesive layer 6 . protective plate 7 may be another disc . in the structure shown in fig1 the laser light for recording , erasing and reproducing the information contained therein is incident in the direction shown by arrow 8 , and has an intensity which is modulated with the information . detection of the recorded information is performed by detecting the reflected light . to produce the two dielectric layers 2 , 4 , the thin film recording layer 3 , and reflecting layer 5 , a sputtering process or other suitable thin film deposition process may be utilized . by way of example , fig2 illustrates the light intensity reflected as a function of time from the surface of the growing film during formation of the first dielectric layer 2 on disc substrate 1 by a sputtering process . as the film deposition progresses ( assuming the refractive index n 2 of the dielectric layer 2 is greater than the refractive index n 1 of the disc substrate 1 ), the reflected light intensity first increases and then reaching a maximum value ( as a result of interference effects ) at a time t 1 , corresponding to a film thickness of approximately λ / 4n 2 , where λ is the wavelength of the incident light . the variation in the reflected light intensity depicted by the graph of fig2 is monitored by an optical detector , and the time t 1 required to reach the maximum value of the reflected intensity is used as a criterion to determine the deposition rate by which the sputtering process is thereafter controlled to complete the deposition of the first dielectric layer 2 and to deposit the second dielectric layer 4 . for example , since the deposition rate r may be written as λ / 4n 2 t 1 , the time t 2 required to deposit a thickness d of the first dielectric layer 2 , i . e . t 2 = d / r = 4n 2 dt 1 / λ may be determined and the deposition continued for a total time equal to t 2 . in the case where the index of refraction n 2 of the dielectric layer 2 is less than the index of refraction n 1 of the disc substrate 1 , the reflectance will initially decrease during deposition and will reach a minimum value when the dielectric layer 2 attains a thickness of λ / 4n 2 . as described above , the deposition rate may thereby be determined and the deposition time controlled for a time t 2 required to deposit a thickness d of the dielectric layer 2 . since the deposition rate of the second dielectric layer 4 is the same as that of the first dielectric layer 2 , the time required to deposit the requisite thickness of the second dielectric layer may also be determined based on the deposition rate . as to the deposition of thin film recording layer 3 , a test substrate is first utilized to determine the deposition rate of the recording layer 3 , and the recording layer 3 is thereafter deposited to the requisite thickness in accordance with that predetermined deposition rate . optical monitoring is not used for controlling the deposition of either the recording layer 3 or the second dielectric layer 4 because their required thicknesses are too thin to generate a maximum or minimum in the monitored light intensity . fig3 more generally shows how a sputtering system would be utilized in accordance with the present invention to form thin film layers of accurate thicknesses by controlling the time for deposition in accordance with the deposition rate initially determined by optical monitoring . fig3 generally shows , in a cross - sectional view , a vacuum chamber 11 having a gas inlet port 12 , and an exhaust port 13 which is connected to a vacuum pump . substrate 14 is attached to a rotatable mount 15 which may be rotated ( as shown by the direction of arrow 17 ) by a motor 16 . facing substrate 14 is a sputtering target 19 which is mounted to electrode 18 . electrode 18 is conventionally driven by a high frequency rf voltage source to produce an electrical discharge in the space between the sputtering target 19 and the substrate 14 . affixed to vacuum chamber 11 by means of a mounting bracket 21 is an optical monitoring device 20 which includes a light source and detector . if the optical property being monitored is the transmitted intensity , rather than the reflected intensity , the optical monitoring device 20 is arranged to have a light source on one side of the substrate 14 , with a separate detector positioned on the other side thereof to monitor the transmitted light . to prevent any film from forming on the back of substrate 14 , a deposition baffle 22 is positioned around the back side of substrate 14 and attached to the vacuum chamber 11 by means of a fixture 23 . in operation , the vacuum chamber 11 is first exhausted to a base vacuum , after which an inert gas such as argon is introduced into the vacuum chamber 11 through the gas inlet port 12 . a high frequency rf voltage is thereafter applied to electrode 18 , causing an electrical discharge to be formed within the vacuum chamber 11 and material from the sputtering target 19 to be ejected by the impact of energetic argon ions produced in the electrical discharge . the material ejected from the sputtering target 19 in this manner deposits on substrate 14 to gradually form a thin film layer . as explained with respect to fig2 accurate control of the film thickness of the first dielectric layer 2 and the second dielectric layer 4 is provided in the preferred embodiment of the invention by controlling the time of sputter deposition , in accordance with the time t 1 needed to reach the maximum or minimum reflected light intensity as measured by the optical monitoring device 20 . since the film thickness of the second dielectric layer 4 is as thin as about 20 nm , a maximum or minimum value in the reflected light intensity is not reached during formation of the second dielectric layer 4 . thus , in order to form the second dielectric layer 4 by a deposition time criterion , an appropriate fraction of the time required to reach the maximum or minimum reflected light intensity during deposition of the first dielectric layer 2 , is utilized as the criterion . this technique greatly improves the accuracy and stability of the film thickness of the second dielectric layer 4 . fig4 schematically shows a second embodiment for a sputtering system , in which a thin film may be simultaneously formed on a large number of disc substrates by utilizing a process control in accordance with the present invention . in fig4 the sputtering system is enclosed within a vacuum chamber 24 having a pallet 25 on which a plurality of disc substrates 26 may be mounted . the pallet 25 is rotatable about shaft 27 . an inert gas such as argon is introduced through inlet port 30 , and is exhausted through exhaust port 31 so that a partial pressure sufficient to sustain an electrical discharge is present within vacuum chamber 24 . an optical monitoring device 32 is positioned by bracket 33 so that light emitted therefrom and reflected from the substrates 26 as they rotate may be detected . as described above , the time required to obtain a predetermined variation in the reflected light intensity is initially determined and a deposition rate is calculated therefrom . the deposition rate is thereafter used as the criterion by which the sputtering time is controlled for deposition of the first and second dielectric layers . with reference to the sputtering system shown in fig4 in which a plurality of disc substrates 26 are mounted on a pallet 25 , a systematic error may be introduced if the optical monitoring device 32 detects light reflected from the rim of the pallet 25 in addition to the light reflected from the disc substrates 26 . thus , in order to properly limit the detected light to only that which is reflected from the disc substrates 26 , the rotation of the pallet 25 is synchronized with respect to the operation of the optical monitoring device 32 , such that only the quantity of light reflected from the disc substrates 26 is detected thereby . to do so , a stepping motor may be utilized to rotate shaft 27 and a synchronization signal derived from the stepping motor may be used to switch on the optical monitoring device 32 at the appropriate times . although the invention disclosed herein has been described with reference to particular embodiments , it is to be understood that these embodiments are merely illustrative of the different aspects and features of the invention . as such , a person skilled in the art may make numerous modifications to the illustrative embodiments described herein , and other arrangements may be devised to implement the disclosed invention which will fall within the spirit and scope of the invention described and claimed herein .