The exposure dosage to get heat focal region satisfies relation: It0.43 = Constant. For expanding the heat focal region the contribution of I is greater than that of t, but the value of k decreases with increase of t. Increase of f and a2 will notably lead to decrease of the heat focal region. Acknowledgement The authors thank the financial supports provided by Natural Science Foundation of China (No. 19934001, 300702171).
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