A lithographic printing plate precursor is disclosed which comprises a support, a photopolymerizable image recording layer and an overcoat which comprises a low-molecular radical inhibitor. After image-wise exposure, the plate is heated whereby the radical inhibitor diffuses from the overcoat to the image recording layer, resulting in an increase of the daylight stability of the exposed and heated precursor. Such plate is especially suitable for on-press processing.

A copolymer includes (i) a plurality of ethylenic moieties A having a structure according to the following formula:

##STR00001## wherein R2 and R3 independently represent hydrogen, a halogen or an optionally substituted linear, branched or cyclic alk(en)yl group, or an optionally substituted aromatic or heteroaromatic group, and (ii) a plurality of acetal moieties B having a structure according to the following formula:

##STR00002## wherein L represents a divalent linking group; x=0 or 1 and R1 represents an optionally substituted aromatic or heteroaromatic group including at least one hydroxyl group and optionally one or more additional substituent(s), and at least one electron withdrawing group in ortho or para position relative to the at least one hydroxyl group.

A recording medium comprises an oleophilic substrate and, thereover, a topmost oleophobic layer comprising a cured polymeric silicone matrix that consists essentially of the addition-cure reaction product of a vinyl-functional polydialkylsiloxane and a silane cross-linking agent. The vinyl-functional polydialkylsiloxane has a molecular weight ranging from 30,000 to 75,000 or 110,000 to 130,000. If the molecular weight of the vinyl-functional polydialkylsiloxane is within the range of 30,000 to 75,000, the molar ratio of silane groups to vinyl groups is within the range of about 11:1 to about 25:1; and if the molecular weight of the vinyl-functional polydialkylsiloxane is within the range of 110,000 to 130,000, the molar ratio of silane groups to vinyl groups is from about 5:1 to about 27:1. The recording medium may be used as a lithographic printing plate.

An on-machine development type planographic printing plate precursor has a support and an image-recording layer on the support. The image-recording layer contains an initiator, an infrared absorber capable of donating electrons to the initiator, and a color-developing substance precursor. The image-recording layer can form an image by exposure to infrared laser, and in a case where the image-recording layer is exposed to an infrared laser with a wavelength of 830 nm at an energy density of 110 mJ/cm.sup.2, a brightness change L of the image-recording layer before and after the exposure is 3.0 or more. A method for preparing a planographic printing plate and a planographic printing method employ the on-machine development type planographic printing plate precursor.

A source of actinic radiation for curing printing plates. The source includes a base with a heat sink and has a length greater than its width. One or more circuit boards mounted on the base provide a plurality of light emitting diodes (LEDs) distributed over the length of the base. A transparent or translucent cover together with the base defines an enclosure for the plurality of LEDs. The source provides the radiation at an emission angle greater than the emission angles of a single LED in a plane perpendicular to the axis of the base, in a plane containing the axis of the base or parallel to the axis of the base and perpendicular to the target illumination plane, or a combination thereof. The source may be configured to replace a fluorescent bulb. Methods and systems for bank exposure of printing plates using such sources are also described.

A source of actinic radiation for curing printing plates. The source includes a base with a heat sink and has a length greater than its width. One or more circuit boards, mounted on a planar mounting surface of the base disposed parallel to the target illumination plane, collectively provide a plurality of light emitting diodes (LEDs) having an ultraviolet center emission wavelength distributed over the length of the mounting surface. A transparent or translucent cover together with the base defines an enclosure for the plurality of LEDs. The heat sink has a heat transfer surface area defined by a plurality of projections with indentations therebetween. The source may be configured to replace a fluorescent bulb.

A lithographic printing plate precursor has a support and an image-recording layer on the support, in which the image-recording layer contains an infrared absorber, a polymerization initiator, and a polymer A. The weight-average molecular weight of the polymer A is more than 15,000 and 150,000 or less, and the ethylenically unsaturated bond valence of the polymer A is 3.0 mmol/g or more.

A positive-working lithographic printing plate precursor including a support and an image-recording layer formed on the support and consisting of a positive-working photosensitive resin composition, in which the positive-working photosensitive resin composition contains a compound having at least one electron-withdrawing group and at least one phenolic hydroxyl group in one molecule, an alkali-soluble resin, and an infrared absorber, and the electron-withdrawing group excludes a sulfonyl group; and a method of preparing a lithographic printing plate using the positive-working lithographic printing plate precursor.