to achieve the aforementioned goals , the methods adopted and the effects thereof are illustrated as follows with exemplary embodiments in accordance with figures . please refer to fig3 and fig4 simultaneously . fig3 is a diagram of compensating focus volt level according to a first embodiment of the present invention . fig4 is a diagram of a variation of side beam added signal ( sbad ) during compensation according to the first embodiment of the present invention . as shown in fig3 , four hundred spokes of a label side 20 ′ of a light - scribe disc 20 are divided into several radial sections , e . g ., twenty spokes as a section , with a total of twenty sections derived . as in the aforementioned prior art , during focus process of each section , sbads are recorded , and referring to the volt magnitude and the spoke corresponding to the maximum sbad during focus process of twenty sections , a preliminary focus volt level curve 21 is approximated . although the preliminary focus volt level curve 21 is close to the shape of the label side 20 ′, a predetermined distance for moving an optical pickup head during focus process is shortened due to increased number of sections . for a part 22 of disc 20 which has no deformation , the preliminary focus volt level curve 21 is still able to provide correct focus volt level . for a concave part 23 of the disc 20 , the volt level from the preliminary focus volt level curve 21 will make the focus point over the label 20 ′, and for a convex part 24 of the disc 20 , the volt level from the preliminary focus volt level curve 21 will make the focus point insufficient to touch the label 20 ′. in both cases , the focus points cannot aim at the label side 20 ′. the present invention utilizes the preliminary focus volt level curve 21 , increasing a volt of a predetermined value δv to form a compensated plus focus volt level curve 25 , or decreasing a volt of a predetermined value δv to form a compensated minus focus volt level curve 26 . as shown in fig4 , for the part 22 without deformation , the compensated plus focus volt level curve 25 adds a volt of the predetermined value δv , lifting the focus point over the label side 20 ′, so the sbad will decrease to a compensated point 22 a . the compensated minus focus volt level curve 26 minused by a volt of the predetermined value δv , pulling the focus point away from the label side 20 ′, so the sbad will decrease to a compensated point 22 b . the preliminary focus volt level curve 21 can still maintain the focus point of the part 22 without deformation on the label side 20 ′, and the sbads are still the maximum . for the concave part 23 , the compensated plus focus volt level curve 25 adds a volt of the predetermined value δv , lifting the focus point away from the label side 20 ′, so the sbad will decrease to a compensated point 23 a instead of the point 23 . the compensated minus focus volt level curve 26 minuses a volt of the predetermined value δv , pulling the focus point close to the label side 20 ′, so the sbad will increase to a compensated point 23 b instead of the point 23 . therefore , the maximum sbad of the concave part 23 is changed . likewise , for the convex part 24 , subtract the compensated minus focus volt level curve 26 from a volt of the predetermined value δv , pushing the focus point away from the label side 20 ′, so the sbad will decrease to a compensated point 24 b instead of the point 24 . the compensated plus focus volt level curve 25 added a volt of the predetermined value δv , lifting the focus point close to the label side 20 ′, so the sbad will increase to a compensated point 24 a instead of the point 24 . therefore , the maximum sbad of the concave part 24 is changed . therefore , the focus point will be more close to the label side 20 ′ due to the new sbads of the concave part 23 and the convex part 24 . compensating in a single direction to increase or decrease a volt of a predetermined value δv can improve the maximum sbad of the concave part 23 or the convex part 24 in the single direction . since the concave part 23 and the convex part 24 of the disc 20 usually occur symmetrically , utilizing bi - directional compensation can simultaneously improve and facilitate correcting of errors . although the present invention adopts bi - directional compensation volt as an example for illustration , this is not supposed to be a limitation of the present invention , and the scope of the present invention includes single direction compensation . this is shown in fig5 , which is a diagram of comparing the maximum sbad of each section according to an embodiment of the present invention . the compensated plus focus volt level curve 25 and the compensated minus focus volt level curve 26 are utilized to perform focus process on the label side , and to detect and record the sbads . then , the sbads from the preliminary focus volt level curve 21 , the compensated plus focus volt level curve 25 and the compensated minus focus volt level curve 26 are compared to derive the maximum sbad in each section . as shown in fig6 , the volt magnitude and the spoke corresponding to the maximum sbad in twenty sections are referred to for approximating a calibrated focus volt level curve 27 to improve the error of the preliminary focus volt level curve 21 of the deformed disc . fig7 is a flowchart of a method of controlling focus on a label side of a light - scribe disc according to a first embodiment of the present invention . the steps of increasing or decreasing a predetermined value δv for a focus volt level curve to improve error of a deformed disc are detailed as follows : firstly , in step p 1 , a label side of the light - scribe disc is divided into a predetermined number of radial sections . in step p 2 , focus process is performed for each section to detect and record sbads . in step p 3 , derive the maximum sbad of each section . next in step p 4 , a focus volt and a spoke corresponding to the maximum sbads derived from each section are utilized to approximate a preliminary focus volt level curve . in step p 5 , a predetermined volt δv for the preliminary focus volt level curve is increased to form a compensated plus focus volt level curve , focus process of the label side is performed to detect and record sbads . in step p 6 , the predetermined volt δv for the preliminary focus volt level curve is decreased to form a compensated minus focus volt level curve , and focus process of the label side is performed to detect and record sbads . in step p 7 , the sbad derived from the preliminary focus volt level curve , the sbad derived from the compensated plus focus volt level curve and the sbad derived from the compensated minus focus volt level curve in each section are compared to derive a maximum sbad in each section . then , in step p 8 , the focus volt and the spoke corresponding to the maximum sbad derived in the step p 7 are utilized to approximate a calibrated focus volt level curve as a focus volt level of the label side . finally in step p 9 , the operation of approximating the focus volt level curve is finished . therefore , the method of controlling focus on a label side of a disc according to the present invention can compensate to increase or decrease a predetermined volt via a preliminary focus volt level curve to thereby form compensated plus or minus focus volt level curves , and , by comparing sbads of the preliminary focus volt level curve with the compensated plus or minus focus volt level curves in each section , the preliminary focus volt level can be calibrated , thereby compensating focus volt level error resulted from the disc deformation . at the same time , the present invention can utilize the compensation of the error of the focus volt level curve to divide the label side of the disc into more sections , thereby deriving a more accurately approximated focus volt level to scribe a clear label pattern . fig8 is a diagram of checking differences of maximum sbads from the method of controlling focus on the label side according to the second embodiment of the present invention . although the reflection rate of the label side is not as good as the data side of the light - scribe disc , they are substantially the same , and therefore the maximum sbad of each section focus aiming at the label side should also be substantially identical . an error status of the focus volt level curve can be checked via a variation of maximum sbads of each section in the approximated focus volt level curve . the more differences between maximum sbads of each section , the less some focus points of certain seriously deformed sections are close to the label side during the process for approximation . the exemplary embodiment sets a threshold δt as a threshold of checking a proper difference among maximum sbads of sections . for those differences among maximum sbads of sections within the threshold δt , the approximated focus volt level curve can be utilized to scribe clear label pattern . for those differences among maximum sbads of sections exceeding the threshold δt , the approximated focus volt level curve needs to be calibrated again . fig9 is a flowchart of a method of controlling focus on a label side of a light - scribe disc according to the second embodiment of the present invention . the steps of checking focus volt level curve to perform calibration are detailed as follows : firstly , in step s 1 , a label side of the light - scribe disc is divided into a predetermined number of radial sections . in step s 2 , the focus process for each section is performed to detect and record sbads . in step s 3 , derive the maximum sbad of each section . next in step s 4 , a focus volt and a spoke corresponding to the maximum sbads derived from the sections are utilized to approximate a preliminary focus volt level curve . in step s 5 , it is checked whether the differences among maximum sbads are within the predetermined threshold δt or not . if the differences among maximum sbads are not within the predetermined threshold δt , the flow goes to step s 6 to increase a predetermined volt δv for the preliminary focus volt level curve , and utilize the compensated plus focus volt level curve to perform focus process of the label side to detect and record sbads . in step s 7 , the predetermined volt δv for the preliminary focus volt level curve is decreased , and the compensated minus focus volt level curve is utilized to perform focus process of the label side to detect and record sbads . then , the flow goes back to step s 3 to compare the sbad derived from the preliminary focus volt level curve , the sbad derived from the compensated plus focus volt level curve and the sbad derived from the compensated minus focus volt level curve in each section to derive a maximum sbad in each section . if the differences among maximum sbads are within the predetermined threshold δt , the flow goes to step s 8 , and sets the preliminary focus volt level curve in step s 4 as a calibrated focus volt level curve . finally in step s 9 , the calibration of the focus volt level curve is finished . fig1 is a diagram of focus controlling method on the label side forming a focus volt level curve for a deformed disc according to an embodiment of the present invention . the solid line in fig1 is a focus volt level curve for moving the pickup head on the label side , and the dotted line is the approximated focus volt level curve for moving the pickup head according to an embodiment of the present invention . via measurement , the maximum error between those two curves is 1 . 3 μm . therefore , the present invention is able to significantly lower the maximum error , i . e ., 2 . 8 μm , resulting from the preliminary focus volt level curve , such that the focus on the label side can be greatly improved . therefore , the method of controlling focus on a label side of a disc according to the second embodiment of the present invention can utilize a threshold to check a maximum difference between sbads of focus volt level curves to determine a proper focus volt level for the label side , and hence improve an efficiency of calibrating the focus volt level . those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention .