now referring to the drawings , preferred embodiments of the invention are described below . a first embodiment of the invention will be described . fig1 is a view of an image forming apparatus to which a led write apparatus embodying the invention is applied . fig1 differs from fig1 in the led write apparatus . a charger 100 opposes a photosensitive drum ( recording medium ) 101 , and uniformly provides the surface of the photosensitive drum with charges . a led write apparatus converts input electric signals to optical on / off signals to which the photosensitive drum 101 is exposed through a lens array 103 . as a result , an electrostatic latent image corresponding to the electric signals input to the led write apparatus 102 is formed on the surface of the photosensitive drum . the photosensitive drum surface where the electrostatic latent image is formed interacts with a developing apparatus 104 to which a bias voltage is applied , and toner is transferred to the photosensitive drum surface corresponding to develop the electrostatic latent image . the photosensitive drum surface developed using the toner is rotated to reach a transcribing portion . a paper sheet 107 is fed from a sheet feeding portion 106 to the transcribing portion , and the toner on the photosensitive drum surface having a polarity reverse to that of the charge of the toner on the photosensitive drum surface , is discharged from a transcription charger 105 to form a latent image on the paper sheet 107 . the image on the paper sheet 107 is not stable because it is electrostatically deposited . a stable image is obtained so that the toner permeates the paper sheet by heating and melting and is fixed by a fixing apparatus 108 . fig2 - 5 are views explaining the first embodiment . fig2 a is a plan view in the plane direction of the photosensitive drum 101 ( the direction along the rotation axis of the photosensitive drum ) and fig2 b is a front view . fig3 is a plan view of a led head , fig4 is a sectional view of the led head , and fig5 a , 5b are a plan and a sectional view of a led head chip . fig6 is a view showing a circuit configuration of the led head chip . the led head write apparatus comprises a led head 2 and a round convex lens 3 , which are held with a frame 4 . the round convex lens 3 is held with a lens holding portion 4a which is a circular hole formed in the frame 4 . in the lens holding portion 4a is a stopper 4b , and the round convex lens 3 is fixed between the stopper 4b and a lens fixing ring 4c . the led head 2 is a one in which an led head chip 21 sealed in a ic package 23 is mounted on a mounting substrate 22 . the led head chip 21 is sealed in the ic package 23 after being connected on a lead frame 25 by bonding wires 26 . a transparent glass 24 is arranged on the front side of the ic package 23 . thereby the light from the led head chip 21 travels through the transparent glass 24 . then the photosensitive drum 101 is exposed to the light through the round convex lens 3 . constitution examples and manufacturing steps of the led head chip 21 will now be described . the led head chip 21 includes 2400 leds corresponding to the lateral width of a4 size and a driving circuit for each led . when being formed on a semiconductor substrate , a led having a width of 30 mm is considered advantageous in terms of cost . accordingly , in this embodiment , 2400 leds 32 are formed on one led substrate ( gaas : gallium arsenide ) 31 having a width of 30 mm ( semiconductor substrate width : 30 μm ). in this case , taking the insulation of the 2400 leds and insulation thereamong into consideration , the four sides of one led 32 comes to about 10 μm ( i . e ., the led width a := 10 μm ). on the other hand , a periphery circuit including a driving circuit for a led is formed on a si substrate 33 measuring about 30 mm × 8 mm . the peripheral circuit comprises 2400 transistors 34 , a latch circuit 35 , a shift register 36 and a bonding pad 37 for connecting an input signal . although the formation procedures for the peripheral circuit are the same as those of a conventional peripheral circuit chip , a space for mounting the led substrate 31 is secured in the vicinity of the transistors 34 . the led substrate 31 is mounted and connected physically on the si substrate 33 on which the peripheral circuit is formed by soldering , welding or the like , and subsequently the peripheral circuit is electrically connected to a terminal of each led 32 on the led substrate 31 . the electrical connection method may be a semiconductor process such as aluminum vapor deposition / etching ( aluminum wiring 38 ). the led head chip 21 is thus formed . subsequently , after being fixed to the lead frame 25 , the led head chip 21 is sealed in the ic package 23 . in the above - mentioned manner , the step of forming the leds 32 on the led substrate 31 and the step of forming the peripheral circuit on the si substrate 33 are separately carried out and subsequently both are physically connected to constitute a led head chip . a peripheral circuit including a driving circuit is formed on the si substrate 33 . thereafter the step of growing a gaas film on the si substrate 33 is carried out to form the led 32 on the gaas film . subsequently the led 32 is electrically connected to the peripheral circuit using aluminum vapor deposition / etching . in this manner , it is possible to carry out all the manufacturing steps of the led head chip in the semiconductor process . because both the led 32 and the peripheral circuit are mounted on one chip , wire bonding is not necessary for connecting the led 32 and the peripheral circuit . the necessary wiring is formed in the semiconductor process , which leads to improved reliability of the wiring and cost saving . further , a gaas substrate is usually expensive in comparison with a si substrate , and therefore , forming not only leds , but also a periphery circuit on the gaas substrate results in increase in cost . accordingly , like the embodiment , when only the leds 32 are formed on the gaas substrate 31 and the periphery circuit is formed on the si substrate 33 , it is possible to manufacture a led head chip economically . of course , if the manufacturing is carried out regardless of cost , the leds and the peripheral circuit may be all formed on the same substrate . thereby , the number of steps carried out in the manufacture of a led head chip are reduced , resulting in easy manufacturing . next , an image writing state with the led write apparatus constituted as above will be described . in image writing , image signals for one line are input as serial signals into the led head chip 21 and stored in the shift register 36 . the image signals for one line are transferred to the latch circuit 35 with latch signals , each output signal is connected to each led 32 via a transistor 34 and driven . in the case where there is sufficient emission time for one line , the output of the latch circuit 35 may be transmitted through a multiplex circuit so that the led 32 is matrix - driven . as a result , time shared driving in which 2400 leds are divided into 8 to 64 blocks may be carried out , which leads to the reduction of internal wiring and driver circuits . the light emitted from the led head chip 21 driven in such manner is transmitted through the round convex lens 3 and imaged on the photosensitive drum 101 . at that time , the width of the light output from the led head chip 21 , which is 30 mm , is magnified on the photosensitive drum up to a necessary image width ( in this example , length of a4 size : 210 mm ). therefore the width of the light outputted from the led head chip 21 is magnified via the round convex lens seven times . at that time , the light from each led is magnified from light having four dimensions of 10 μm to one having four dimensions of 70 μm . an example of constitutional specifications ( e . g ., a lens position , a focal distance for such magnification ) is as follows : distance between led head chip and round convex lens : 26 mm distance between round convex lens and photosensitive drum : 182 mm subsequently , while the photosensitive drum is rotated in the arrow direction of fig2 b , the 2400 leds on the led head chip 21 are turned on / off according to the input signal . thereby an image ( electrostatic latent image ) having a width of 210 mm is formed on the photosensitive drum 101 . now a second embodiment will be described . the second embodiment differs from the above - mentioned first embodiment in shape of leds on a led head chip and in lens constitution guiding the light outputted from the led head chip to a photosensitive drum . fig7 - 11 are views for explaining the second embodiment . fig7 is a plan view of a led head chip . fig8 a is a plan view showing a constitution of a photosensitive drum 101 in a plane direction ( a direction along its rotation axis ), and fig8 b is a front view . fig9 is a perspective view showing an arrangement of a led head , a lens , and a photosensitive drum . fig1 a , 10b are views showing how light travels in the embodiment of the invention . fig1 is a view showing a constitution of a holding part of a second lens for guiding the light from the led head chip to the photosensitive drum . the parts corresponding to each other in fig2 a , 2b , and 3 are represented by the same reference numerals . as shown in fig7 in a led head chip 51 is arranged a led substrate 52 made of a gaas semiconductor substrate on the si substrate 33 and leds 53 are formed on the led substrate 52 . the led 53 of the embodiment is formed so as to have a rectangular shape in which its length is longer than its width &# 34 ; a &# 34 ; in the led arrangement direction . by employing such a rectangular shape for the led , an emission area nearly equal to that of a conventional led ( four sides of 60 μm ) can be obtained and as a result a sufficient quantity of emission light can be obtained . the led having four sides of 10 μm of the above embodiment is not well suited to a high speed image forming apparatus because of insufficient quantity of emission light . to make the leds applicable to a high speed image forming apparatus , the shape of the leds is enlarged in the length b direction to obtain a large emission area . the shape of the led is not limited to be rectangular , and for example , an oval led may be applied . in conclusion , it is a requisite only that the length b is longer than the width a . now constitution examples will be described in the case of a rectangular led 53 . when 2400 leds 53 are formed on the led substrate 52 , the width a of each led 53 is required to be about 10 μm in relation to the dimensions of such a semiconductor substrate that can be formed as one chip . accordingly , in order to obtain the emission area nearly equal to that of a led having four sides of 60 μm , the length b is selected to be about 360 μm . thereby a sufficient amount of emission light equivalent to the led having four dimensions of 60 μm can be obtained . the light emitted from one led corresponds to one dot in image formation . it is preferable that the ratio of length to width of one dot is 1 : 1 because when the ratio exceeds 1 : 1 . 5 , image quality is degraded . as mentioned above , in the case where the shape of the led 53 is selected to be rectangular and measure 10 μm × 360 μm , the ratio of length and width comes to 1 : 36 , and which is unpractical . for that reason , the light in the length b direction is reduced via a lens . as shown in fig8 a , 8b , and 9 , in the embodiment are a first lens 54 and a second lens 55 is arranged between the led head 2 and the photosensitive drum 101 . the first and second lenses are plate - shaped and have a degree as a lens in only one direction , namely either its length or width direction . the first lens 54 is arranged in the vicinity of the led head 2 to magnify the light outputted from the led head chip 51 and project on the photosensitive drum 101 . on the other hand , the second lens 55 is arranged in the vicinity of the photosensitive drum 101 to reduce the light output from the led head chip 51 and project on the photosensitive drum 101 . the magnification of the first lens 54 is set to be about 7 from the relation to the size of an image to be formed ( 210 mm length of a4 ) and the magnification of the second lens 55 is set to be about 1 / 5 . 9 from the relation to the above width to be magnified . thereby the size of one dot formed on the photosensitive drum 101 comes to about 70 μm × 61 μm . as a result , the light output from the led head travels as shown in fig1 a , 10b . fig1 a , 10b show a traveling path of light in the led write apparatus in the direction of the plane of the photosensitive drum 101 ( in the direction along the rotation axis of the drum ) and the traveling path on the front side of the photosensitive drum 101 , respectively . the light outputted from the led head 2 is magnified in the plane direction seven times via the first lens 54 and projected on the photosensitive drum 101 . on the other hand , in the front side direction of the photosensitive drum 101 ( image scanning direction ), the light outputted from the led head chip 51 is reduced 1 / 5 . 9 times via the second lens 55 and projected on the photosensitive drum 101 . accordingly , a constitution example of the lens arrangement as shown in fig9 a and 10b , is as follows : distance between led head chip and first lens : 26 mm focal distance of first lens on &# 34 ; degree &# 34 ; side : 22 . 75 mm the first lens 54 is held by a frame 6 which holds the led head 2 . in this case , while the lens holding part is rectangular , the holding method is the same as that shown in fig2 namely the first lens is held by holding between a stopper 6b integrally formed in the frame 6 and a fixing member 6c inserted into a lens holding member 6a . for example , as shown in fig1 , the frame 71 is planted at a rotation axis 101a of the photosensitive drum 101 and the second lens 55 is fixed to the frame 71 . thereby the distance between the second lens 55 and the photosensitive drum 101 can be precisely set . now the method to improve the sensitivity of the photosensitive drum will be described . the opc photosensitive drum has a constitution as shown in fig1 , 14 . fig1 shows a single layer type photosensitive drum , and reference numeral 131 represents a part of a cylindrical aluminum tube , on which a carrier generating member ( cgm ) which generates a carrier by exposure to light and a carrier transporting member ( ctm ) 143 which transports the carrier are double - coated , which have thicknesses of 2 to 3 μm and 20 to 30 μm , respectively . at the present level of technology , a function separated type is usually used with a few exceptions from the aspects of mass - productivity , stability and basic characteristics . as photosensitive materials for a function separated type photosensitive drum are used azo pigments for cgm and hydrazone derivatives for ctm . the optical sensitivity of the photosensitive drum comes to about 0 . 3 to 1 μj / cm 2 in general , and it is known that this sensitivity can be improved by optimization of the combination of cgm and ctm materials ( since the materials are organic materials , feasible material combinations are indefinite ), the improvement of purity of the materials , the enhancement of concentration of the cgm material , and thin - layer coating . at present , although the sensitivity of a high sensitive photosensitive drum is about 0 . 1 μj / cm 2 it is possible to improve up to about 0 . 025 μj / cm 2 . a . correction of lowered light intensity in the circumferential portion of lens in a usual optical system , as shown in fig1 , the circumferential portion of the lens has a tendency to be dark in comparison with the center the necessity of the correction depends on the selected lens . particular , this tendency is remarkable in the case of a lens with a small aperture . in the case where the tendency is remarkable , a circuit for controlling the emission current of an integrated light emission element is used , more particularly , a current control resistor 40 is inserted , as shown in fig1 , between the led 32 and the control transistor 34 in the circuit of fig5 to control the current and increase the current value as it goes from the center portion to the circumferential portion , more specifically to correct by decreasing the resistance value of the current control resistor 40 as it goes from the center portion to the circumferential portion . the following are known as aberrations in a lens optic system , which are improved as follows . the invention relates to a led light source and monochromatic light , and therefore the chromatic aberration is not caused theoretically . ideally speaking , the shape of a lens should be spherical . in practical , however , the lens is manufactured to be nearly spherical because of manufacturing easiness and as a result spherical aberration is caused . because of the spherical aberration , the formed image gets blurred as it goes from the center portion of the lens to the circumferential portion . in order to solve the problem , a high precision lens as spherical as possible is used , only the center portion of a large aperture lens is used , or a combination of plural lenses is used . the processing accuracy of a lens is the cause of the aberration . particularly , when the axial symmetry of the curved lens surface is problematic , the aberration is caused . to enhance the working accuracy is only a remedy against the problem . the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the present embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein .