短波紫外反射法广泛应用于潜在指印的显现,但对香烟盒外包装塑料薄膜上的潜在指印却无法得到较好的显现效果。香烟盒外包装塑料薄膜是一种无色透明且厚度薄的双轴拉伸聚丙烯（BOPP）薄膜,薄膜上的潜在指印往往较为微弱。短波紫外反射方法直接显现薄膜上的潜在指印时,大部分短波紫外光穿透了薄膜并被薄膜后面的物体反射,反射后的短波紫外光再次穿透薄膜,无法形成客体与痕迹之间有效的反差。本文在实验基础上提出了相应的改进方法,利用强吸收短波紫外介质完全吸收穿透薄膜的短波紫外光,使得穿透薄膜的短波紫外光不被薄膜后的物体反射从而突出显现存在于薄膜上的潜在指印痕迹。该方法通过控制照射时间可以最大限度的保持物证检材的原始状态,减少对后续DNA检验的影响。

When cigarette wrapping film is made of Biaxially Oriented Polypropylene (BOPP), which is colorless, transparent and very thin, the “502” fuming method, a conventional way to develop the latent fingerprint on cigarette wrapping film, is not recommended for development of fingerprint on such film because of the steadfast adherence of the fingerprint’s ingredients onto the film surface over heat, negatively affecting the following DNA examination. Hence, the optical inspection is the most preferential choice for its non-destructiveness on the latent fingerprint development. The shortwave UV reflection photography is now widely applied to develop the latent fingerprint on a variety of substrates except the BOPP film unless a modification. The colorlessness, transparency and thinness of the film are found to be the main cause. When photograph being taken, most short-wave UV lights transmitting through the film are reflected back by the object behind the film to make the lights cross the film reversely. The reflected lights are stronger than the diffuse reflection lights produced by the latent fingerprint on cigarette wrapping film, leading a failure to form an effective contrast between the substrate and fingerprint. To solve this problem, this paper proposes a modified method by utilization of some strong short-wave UV absorbing medium such as print paper, thick glass and other similar materials, to let the transmitted UV lights completely absorbed behind the film. Therefore, the lights received by the UV camera or CCD are only the left diffuse reflection lights emitted by the latent fingerprint on cigarette wrapping film. Without the disturbance of the reflected lights, the latent fingerprint on the film can be obviously distinct under shortwave UV light radiation. Whereas be cautious to the irradiation time, otherwise the over-exposure to the radiation might hinder the subsequent DNA analysis.