CN102686407B - 高对比度逆向反射片材和车牌 - Google Patents
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Abstract
本发明涉及形成高对比度、非波长依赖的逆向反射片材，其中通过在所述逆向反射片材的至少一部分上包括光散射材料制成所述高对比度、非波长依赖的逆向反射片材。所述光散射材料降低了逆向反射片材的亮度，而在散射光下观看时不会显著改变所述逆向反射片材的外观。
Description
技术领域
本发明整体涉及一种高对比度逆向反射片材和车牌，制造所述高对比度逆向反射片材和车牌的方法以及能够阅读所述高对比度逆向反射片材和车牌的自动车牌阅读器***。
背景技术
自动车辆识别（AVR）是用来表示通过电子***检测和识别车辆的术语。示例性的AVR用途包括自动收费、交通法规实施、搜索与犯罪关联的车辆以及设施进入控制。理想的AVR***是通用的（即，它们能够100%准确地阅读所有的车牌）。现在使用的AVR***的两种主要类型是（1）利用RFID技术阅读附装于车辆的RFID标签的***和（2）利用机器或装置阅读附着在车辆上的机器可读代码的***。
RFID***的优点之一在于它们的高准确率，其可借助于RFID标签上的检错和纠错信息来实现。利用熟知的数学方法（例如循环冗余检验或CRC），可确定阅读正确（或错误）的概率。然而，RFID***也有一些缺点，其中包括并非所有车辆都有RFID标签。另外，现有的不加电的“无源”RFID标签阅读器在精准地确定物体的确切位置方面存在困难。相反，它们只是报告在它们的敏感区域内存在或不存在标签。此外，许多RFID标签阅读器仅在短距离内工作，在存在金属的情况下表现很差，并且在有许多带标签的物体存在时会受到干扰而阻塞。这些问题的一些可通过使用有源RFID技术或类似方法得到解决。然而，这些技术需要昂贵的、耗电的电子器件和电池，并且它们当附着在致密或金属物体上时仍然不能准确地确定位置。
机器视觉***（通常称作自动车牌阅读器或ALPR）使用机器或装置来阅读附着在车辆上的机器可读代码。在许多实施例中，车牌上附着、印刷或邻接有机器可读代码。一个示例性ALPR***在图1中示意地示出，图1示出了照明和观看逆向反射标签的过程。如本文所用，术语“逆向反射”是指这样一种属性，具备该属性的物体可使斜入射光线的反射方向反向平行于或大致反向平行于其入射方向，从而使得光线返回到光源或紧邻光源的位置。红外光源106照明布置在车牌104上的逆向反射标签102。逆向反射标签102将光源106发射的红外光直线反向反射到红外光源106，在该处反射光被诸如（举例来说）红外相机之类的红外传感器108俘获。ALPR***的一个优点是它们几乎到处可以被使用，这是因为在世界的几乎所有地区都要求车辆具有其上具有可视觉识别的信息的车牌。然而，识别视觉标签的任务可能是复杂的。例如，ALPR***的阅读准确度在很大程度上取决于阅读器读取时被俘获图像的质量。现有***难以将标签从复杂背景区分开，而且难以处理可变光照。另外，当车牌变模糊或脏污时，ALPR***的准确度下降。
造出用于ALPR***的高对比度车牌的现有技术的方法涉及这样的材料，其吸收红外波长范围内的光发射可见光波长范围内的光。例如，美国专利No.6,832,728描述的车牌包括透射可见光、不透射红外光的标记。美国专利公布No.2007/139775描述的车牌包括阻挡红外光的材料，这在车牌上产生对比度。美国专利No.3,758,193描述了透射红外光、吸收可见光的材料，其用于逆向反射片材上。
发明内容
本发明人认识到需要在红外波长范围内具有高对比度、同时在可见光波长范围内保持逆向反射片材的颜色特征的逆向反射片材。本发明人还认识到需要非波长依赖的、高对比度逆向反射片材。本发明人还认识到需要包括这种片材的车牌以及能够检测这种车牌的OCR和/或ALPR***。
本发明人认识到可通过在所述逆向反射片材的至少一部分上包括光散射材料制成所述高对比度、非波长依赖的逆向反射片材。所述光散射材料降低了所述逆向反射片材的亮度，同时当在散射光下观看时不会显著改变所述逆向反射片材的外观。
本发明的一些优选实施例涉及逆向反射片材，其包括含有光散射材料的标记。本发明的其它优选实施例涉及一种包括上述逆向反射片材的车牌。本发明的其它优选实施例涉及一种光学字符识别光学字符识别***，其包括：光学字符识别相机；光源，其发射与光学字符识别相机同轴的红外光；以及车牌，其包括上述逆向反射片材。
附图说明
图1是现有技术的ALPR***的示意图。
图2A是在漫反射可见光中看到的车牌的照片。
图2B是在逆向反射红外光中看到的图2A的车牌的照片。
图3A是在漫反射可见光中看到的车牌的照片。
图3B是在逆向反射红外光中看到的图3A的车牌的照片。
图4A是在漫反射可见光中看到的车牌的照片。
图4B是在逆向反射红外光中看到的图4A的车牌的照片。
具体实施方式
本发明的***、方法和设备一般地描述了在逆向反射片的至少一部分上包括光散射材料。光散射材料降低了逆向反射片材的亮度，并且当在散射光下观看时不会显著改变逆向反射片材的外观，从而产生高对比度、非波长依赖、逆向反射的片材。现有的车牌已包括红外光吸收、阻挡、不透明和透射的材料，然而没有一种车牌包括光散射材料来产生高对比度。光散射材料的使用具有在现有的红外光吸收、阻挡、不透明和透射的材料之上某些优点。例如，包括光散射材料的车牌的对比度是非波长依赖的，这是因为利用光散射材料形成的标记在逆向反射中具有高对比度，但是在正常（散射）光条件下观看的车牌会仍然具有无论何种赏心悦目的外观。这种非波长依赖性使得在选择用于阅读包括光散射材料的车牌的OCR或ALPR***中的相机的过程中能够具有更大的柔性。此外，虽然找到吸收红外光但以可见的散射光显示颜色的材料是困难并且昂贵的，然而要找到散射红外光且在散射光下观看时不显著改变逆向反射片材的外观的材料却较为容易且并不昂贵。
光散射材料优选地用于在逆向反射片材或车牌上形成标记。示例性标记包括例如车牌号或标识符。光散射材料可为表面散射材料或可为体散射材料。在使用体散射材料时，其可被包括在标记油墨中或可置于标记油墨下。如果光散射材料置于所述标记油墨下，则优选的是采用与基底（例如，逆反射片材）相同的颜色以使得其在散射光中不改变所述标记的外观。示例性红外光散射材料包括例如油墨、墨粉、染料和胶带。当使用油墨、墨粉或染料时，它们优选地是平坦（而非粗糙）的，这是因为粗糙的油墨、墨粉和染料可能并不充分散射红外光。作为另外一种选择，在想要散射光的区域中，车牌可简单地选择性地通过例如磨砂或喷珠而***糙。在这类具体实施中，车牌的顶面充当光散射材料。
其上印刷有光散射材料的车牌的一些示例性实施例示于图2A–4B中。
图2A是在可见光中看到的现有技术的lowa车牌200的照片。车牌200包括标记202（“SAM PLE”），在散射的可见光中人眼可见所述标记。标记202使用Digital License Plate热色带车牌打印机（3M公司出售）利用彩色油墨（此例中为深蓝色TTR1301（3M公司出售））印刷在逆向反射片材204上。在散射光照条件下产生2.25:1的对比度。图2B是图2A的车牌在逆向反射的红外光下的照片。
图3A是当在散射可见光中观看时的lowa车牌300的照片。车牌300包括用光散射材料印刷的标记302（“SAM PLE”）。具体地讲，标记302是使用Digital License Plate热色带车牌打印机（3M公司出售）印刷的。首先，光散射白色油墨（例如，DPL热转印色带，白色TTR1321，可得自3M公司）被印刷在逆向反射片材304上。接着，在光散射白色油墨上印刷彩色油墨（此例中为DLP热转印色带深蓝色TTR1301，可得自3M公司）。标记302在漫射可见光中可为人眼所见。图3B是图3A的车牌在逆向反射的红外光下的照片。白色油墨能散射红外光，因此在逆向反射光照条件下观看时标记302呈现黑色。当在逆向反射的红外光下观看所述车牌时，观看者会立即注意到标记302和逆向反射片材304之间的高对比度（约20:1）。此外，这种对比度显著高于现有技术车牌中显示的对比度。增加的对比度导致当所述车牌通过ALPR***读取时更高的车牌阅读准确率。
数字图像的对比度是适当选择的亮区域的灰度值对于适当选择的暗区域的灰度值之比。理论上，物体的对比度与该物体的图像的对比度相同。然而，光照条件需要加以规定并且需要小心地控制曝光。亮区须不使检测器饱和，并且暗区须充分位于噪声电平上方以使噪声不显著影响测量结果。因为这些理由，难以（有时不能）根据单个图像测量高对比度物体的对比度。8位相机可理论上测量高达256:1的对比度。然而，如果噪声电平为约10个计数，则人们应当优选使用约30个计数的最小测量结果。为了避免图像饱和，人们应当优选使用约200个计数的最大值。这将单个图像的最大对比度限制为200/30或约6.7:1。为了克服这些问题，人们可使用在不同的仔细控制的照明条件和曝光设置下的多幅图像，以扩展测量的动态范围。
图4A是Minnesota车牌400在漫射可见光中观看时的照片。车牌400包括标记402（“123 ABC”）和位于车牌400的右下角的注册贴签404。注册贴签404包括光散射材料。具体地讲，注册贴签404包括（1）在逆向反射红外光中可见的条形码形式的机器可读信息（形成这种条形码的方法在例如与本文一同提交的美国专利申请No.65853US002和No.65859US002中更详细地描述）；（2）彩色油墨（此例中为红色SharpieTM永久性标记）用于在注册贴签上形成数字“10”其在散射可见光中裸眼可见；和（3）几片透明的牌胶带（光散射材料）覆盖于形成2D条形码的注册贴签上。图4B是在逆向反射红外光中观看时的图4A的车牌的照片。车牌和注册贴签在正常（漫射）光照条件下观看时保持所有其将信息传递给阅读者的现有功能，此外，在逆向反射光照条件下观看时数字信息可被条形码阅读器获得。
本发明的一些优选实施例还描述了能够检测上述逆向反射片材和车牌的OCR和/或ALPR***。如上所述，一个示例性OCR或ALPR***可包括OCR相机（例如，Federal Signal Company的分公司PIPS Technology销售的832型SpikeTM）、红外光源以及上述车牌。光源优选地发射与OCR相机的取景器几乎同轴的光。
对由端值限定的所有数值范围的表述旨在包括归入该范围内的所有数字（即，1至10的范围包括例如1、1.5、3.33、和10）。
本领域的技术人员将会知道，可以在不脱离本发明基本原理的前提下对上述实施例和实施方式的细节做出多种更改。此外，在不脱离本发明的精神和范围的前提下，对本发明的各种修改和更改对本领域技术人员将是显而易见的。因此，本专利申请的范围应当仅由以下权利要求书确定。
Claims (8)
1.一种逆向反射片材，其包括：
标记，其包括光散射材料。
2.根据权利要求1所述的逆向反射片材，其中所述光散射材料是表面散射材料和体散射材料之一。
3.根据权利要求1所述的逆向反射片材，其中所述光散射材料是油墨、墨粉、染料和胶带之一。
4.根据权利要求1所述的逆向反射片材，其中所述光散射材料是已被有选择地粗糙化的表面。
5.根据权利要求1所述的逆向反射片材，其中所述光散射材料是彩色、透明和白色光散射材料之一。
6.根据权利要求1所述的逆向反射片材，其中所述光散射材料位于所述标记下、所述标记内或所述标记上。
7.一种包括根据权利要求1-6中任一项所述的逆向反射片材的车牌。
8.一种光学字符识别***，其包括：
光学字符识别相机；
光源，其发射与所述光学字符识别相机同轴的红外光；以及
车牌，其包括根据权利要求1-6中任一项所述的逆向反射片材。
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| Application Number | Priority Date | Filing Date | Title |
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| US24976509P | 2009-10-08 | 2009-10-08 | |
| US61/249,765 | 2009-10-08 | ||
| PCT/US2010/051507 WO2011044149A1 (en) | 2009-10-08 | 2010-10-05 | High contrast retroreflective sheeting and license plates |
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| CN102686407A CN102686407A (zh) | 2012-09-19 |
| CN102686407B true CN102686407B (zh) | 2015-07-08 |
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| EP (1) | EP2485899B1 (zh) |
| CN (1) | CN102686407B (zh) |
| BR (1) | BR112012007870A2 (zh) |
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| Publication number | Priority date | Publication date | Assignee | Title |
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- 2010-10-05 CN CN201080044782.9A patent/CN102686407B/zh not_active Expired - Fee Related
- 2010-10-05 BR BR112012007870-4A patent/BR112012007870A2/pt not_active Application Discontinuation
- 2010-10-05 EP EP10822548.3A patent/EP2485899B1/en not_active Not-in-force
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| CN102686407A (zh) | 2012-09-19 |
| US20150353029A1 (en) | 2015-12-10 |
| US9937879B2 (en) | 2018-04-10 |
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| EP2485899A1 (en) | 2012-08-15 |
| US9145098B2 (en) | 2015-09-29 |
| EP2485899B1 (en) | 2017-12-27 |
| MX2012004107A (es) | 2012-05-08 |
| BR112012007870A2 (pt) | 2020-08-18 |
| WO2011044149A1 (en) | 2011-04-14 |
| US20120195470A1 (en) | 2012-08-02 |
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