Photothermographic material and image forming method

1 . A photothermographic material comprising a support having on one surface thereof a photosensitive silver halide, a non-photosensitive organic silver salt, a reducing agent and a binder, and satisfying a relationship of image tone defined by the following Expression (1):

( a* 121t −a* 117t ) 2 +( b* 121t −b* 117t ) 2 <2,  (1)

wherein a* 121t and b* 121t each represent a value of CIELa*b* obtained by thermal development at a developing temperature of 121° C. for t (sec), and a* 117t and b* 117t each represent a value of CIELAB obtained by thermal development at a developing temperature of 117° C. for t (sec), said values occurring at an optical density of 1.2; and t represents a duration (sec) required to attain a maximum density when the material has been exposed to sufficient light for producing Dmax and is developed at 121° C.

2 . The photothermographic material according to claim 1 , which satisfies a relationship of image tone defined by the following Expression (2):

( a* 121t −a* 117t ) 2 +( b* 121t −b* 117t ) 2 <1.5,  (2)

wherein a* 121t and b* 121t each represent a value of CIELa*b* obtained by thermal development at a developing temperature of 121° C. for t (sec), and a* 117t and b* 117t each represent a value of CIELAB obtained by thermal development at a developing temperature of 117° C. for t (sec), said values occurring at an optical density of 1.2; and t represents a duration (sec) required to attain a maximum density when the material has been exposed to sufficient light for producing Dmax and is developed at 121° C.

3 . The photothermographic material according to claim 1 , which satisfies a relationship of image tone defined by the following Expression (3):

( a* 121t −a* 121(t−3) ) 2 +( b* 121t −b* 121(t−3) ) 2 <2,  (3)

wherein a* 121t and b* 121t each represent a value of CIELAB obtained by thermal development at a developing temperature of 121° C. for t (sec), and a* 121(t−3) and b* 121(t−3) each represent a value of CIELa*b* obtained by thermal development at a developing temperature of 121° C. for t−3 (sec), said values occurring at an optical density of 1.2; and t represents a duration (sec) required to attain a maximum density when the material has been exposed to sufficient light for producing Dmax and is developed at 121° C.

4 . The photothermographic material according to claim 1 , which satisfies a relationship of image tone defined by the following Expression (4):

( a* 121t −a* 121(t−3) ) 2 +( b* 121t −b* 121(t−3) ) 2 <1.5,  (4)

wherein a* 121t and b* 121t each represent a value of CIELAB obtained by thermal development at a developing temperature of 121° C. for t (sec), and a* 121(t−3) , and b* 121(t−3) each represent a value of CIELa*b* obtained by thermal development at a developing temperature of 121° C. for t−3 (sec), said values occurring at an optical density of 1.2; and t represents a duration (sec) required to attain a maximum density when the material has been exposed to sufficient light for producing Dmax and is developed at 121° C.

5 . The photothermographic material according to claim 3 , wherein all of the values for a* 121t , b* 121t , a* 117t , b* 117t , a* 121(t−3) , and b* 121(t−3) are negative.

6 . The photothermographic material according to claim 4 , wherein all of the values for a* 121t , b* 121t , a* 117t , b* 117t , a* 121(t−3) , and b* 121(t−3) are negative.

7 . The photothermographic material according to claim 1 , wherein a value for a* 121t is −2 or less.

8 . The photothermographic material according to claim 3 , wherein a value for a* 121t is −2 or less.

9 . The photothermographic material according to claim 1 , wherein the non-photosensitive organic silver salt has a silver behenate content of 35 to 85 mol %.

10 . The photothermographic material according to claim 1 , wherein the reducing agent is a hindered-type reducing agent or a bisphenol-type reducing agent.

11 . The photothermographic material according to claim 3 , wherein the reducing agent is a hindered-type reducing agent or a bisphenol-type reducing agent.

12 . The photothermographic material according to claim 1 , further comprising a development accelerator.

13 . The photothermographic material according to claim 1 , further comprising a hardening agent.

14 . An image forming method comprising exposing a photothermographic material to light and carrying out thermal development to form an image, wherein the material comprises a support having on one surface thereof a photosensitive silver halide, a non-photosensitive organic silver salt, a reducing agent and a binder, and satisfies a relationship of image tone defined by the follwing Expression (1):

( a* 121t −a* 117t ) 2 +( b* 121t −b* 117t ) 2 <2,  (1)

wherein a* 121t and b* 121t each represent a value of CIELa*b* obtained by thermal development at a developing temperature of 121° C. for t (sec), and a* 117t and b* 117t each represent a value of CIELAB obtained by thermal development at a developing temperature of 117° C. for t (sec), said values occurring at an optical density of 1.2; and t represents a duration (sec) required to attain a maximum density when the material has been exposed to sufficient light for producing Dmax and is developed at 121° C.

15 . The image forming method according to claim 14 , wherein a thermal developing duration is 15 sec or less.

16 . The image forming method according to claim 15 , wherein a thermal developing duration is 7 to 15 sec.

17 . The image forming method according to claim 14 , wherein light exposure is conducted by an He—Ne laser, a red semiconductor laser, an He—Cd laser or a blue semiconductor laser.

18 . The image forming method according to claim 14 , wherein thermal development is carried out at a temperature of 100 to 140° C.

19 . The image forming method according to claim 14 , wherein thermal development is carried out at a temperature of 110 to 130° C.

20 . The image forming method according to claim 14 , wherein thermal development is carried out using a drum-type heater or plate-type heater.