因(yin)為(wei)印刷工藝決定(ding)了印刷只能(neng)采用網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)再現(xian)(xian)(xian)原稿的(de)(de)(de)(de)連續(xu)(xu)調層(ceng)次(ci)，圖(tu)若放大(da)(da)(da)看，就(jiu)(jiu)會(hui)發現(xian)(xian)(xian)是(shi)由(you)無數個大(da)(da)(da)小(xiao)(xiao)不(bu)等(deng)的(de)(de)(de)(de)網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)組成(cheng)的(de)(de)(de)(de)。我們看到網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)大(da)(da)(da)小(xiao)(xiao)雖然不(bu)同(tong)，但都占據同(tong)等(deng)大(da)(da)(da)小(xiao)(xiao)的(de)(de)(de)(de)空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)，這是(shi)因(yin)為(wei)原稿圖(tu)像一經加網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)以后，就(jiu)(jiu)把圖(tu)像分割(ge)成(cheng)無數個規則排(pai)列的(de)(de)(de)(de)網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)，即(ji)把連續(xu)(xu)調圖(tu)像信息變成(cheng)離散的(de)(de)(de)(de)網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)圖(tu)像信息。網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)越大(da)(da)(da)，表(biao)(biao)現(xian)(xian)(xian)的(de)(de)(de)(de)顏(yan)色(se)越深，層(ceng)次(ci)越暗；網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)越小(xiao)(xiao)，表(biao)(biao)現(xian)(xian)(xian)的(de)(de)(de)(de)顏(yan)色(se)越淺，表(biao)(biao)示(shi)的(de)(de)(de)(de)層(ceng)次(ci)越亮。每個網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)占有的(de)(de)(de)(de)固定(ding)空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)大(da)(da)(da)小(xiao)(xiao)是(shi)由(you)加網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)線(xian)數決定(ding)的(de)(de)(de)(de)，例如(ru)，加網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)目數為(wei)150lpi，則在一英(ying)寸的(de)(de)(de)(de)長度(du)或寬(kuan)度(du)上(shang)有150個網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)。網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)空(kong)(kong)間(jian)(jian)的(de)(de)(de)(de)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)和(he)網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)大(da)(da)(da)小(xiao)(xiao)是(shi)兩個不(bu)同(tong)的(de)(de)(de)(de)概念，例如(ru)C50%代表(biao)(biao)的(de)(de)(de)(de)含義(yi)是(shi)網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)大(da)(da)(da)小(xiao)(xiao)占網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)的(de)(de)(de)(de)50%，100%是(shi)指網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)大(da)(da)(da)小(xiao)(xiao)全(quan)部覆蓋網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)，即(ji)印刷中(zhong)所(suo)稱的(de)(de)(de)(de)"實地"，0%由(you)于沒(mei)有網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)，只有網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)，所(suo)以這塊(kuai)地方就(jiu)(jiu)沒(mei)有油墨被(bei)印上(shang)。顯然掛網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)目數越大(da)(da)(da)，網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)點(dian)(dian)(dian)(dian)(dian)(dian)(dian)所(suo)占空(kong)(kong)間(jian)(jian)位(wei)(wei)(wei)(wei)置(zhi)(zhi)(zhi)(zhi)越小(xiao)(xiao)，能(neng)描述的(de)(de)(de)(de)層(ceng)次(ci)就(jiu)(jiu)越多，越細膩。事實上(shang)，原稿的(de)(de)(de)(de)層(ceng)次(ci)和(he)色(se)彩(cai)就(jiu)(jiu)是(shi)通過(guo)這種掛網(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)(wang)的(de)(de)(de)(de)方法被(bei)再現(xian)(xian)(xian)出來的(de)(de)(de)(de)。