<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>0011 - Disclosure - 4a. DEVELOPMENT OF SPECTRAL PATTERN MATCHING TECHNOLOGY</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>1</Level><ElementName>VSUL_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>VSUL_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Notes to Financial Statements</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>
</LabelSeparator><Level>2</Level><ElementName>us-gaap_InProcessResearchAndDevelopmentPolicy</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="From2012-10-01to2013-06-30" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&lt;b&gt;The Company&amp;#146;s ChromaID&amp;#153; Technology&lt;/b&gt;&lt;/p&gt;
&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company has invented a way to project light
at a material (solid surface, liquid, or gas) and measure the amount of light that is reflected back. The pattern of this reflected
light is compared to other patterns the Company has captured and this allows the Company to identify, detect, or diagnose materials
that cannot be identified by the human eye. The Company refers to this pattern of reflected light as a ChromaID&amp;#153;. The Company
designs ChromaID scanning devices made with electronic, optical, and software parts to produce and capture the light.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company&amp;#146;s first product, the ChromaID
F12 Lab Kit, scans and identifies solid surfaces. The Company is marketing this product to customers who are considering licensing
the technology. Target markets include, but are not limited to, commercial paint manufacturers, pharmaceutical equipment manufacturers,
process control companies, currency paper and ink manufacturers, security card, reader, and scanner manufacturers, food processing,
and electronic gaming.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&lt;b&gt;The Company&amp;#146;s Patents&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On August 9, 2011, the Company was issued US
Patent No. 7,996,173 B2 entitled &amp;#147;Method, Apparatus and Article to Facilitate Distributed Evaluation of Objects Using Electromagnetic
Energy,&amp;#148; by the United States Office of Patents and Trademarks. The patent expires August 24, 2029.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On December 13, 2011, the Company was issued
US Patent No. 8,076,630 B2 entitled &amp;#147;System and Method of Evaluating an Object Using Electromagnetic Energy&amp;#148; by the
United States Office of Patents and Trademarks. The patent expires November 7, 2028.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On December 20, 2011, the Company was issued
US Patent No. 8,081,304 B2 entitled &amp;#147;Method, Apparatus and Article to Facilitate Evaluation of Objects Using Electromagnetic
Energy&amp;#148; by the United States Office of Patents and Trademarks. The patent expires July 28, 2030.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On October 9, 2012, the Company was issued US
Patent No. 8,285,510 B2 entitled &amp;#147;Method, Apparatus, and Article to Facilitate Distributed Evaluation of Objects Using Electromagnetic
Energy&amp;#148; by the United States Office of Patents and Trademarks. The patent expires July 31, 2027.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On February 5, 2013, the Company was issued
US Patent No. 8,368,878 B2 entitled &amp;#147;Method, Apparatus and Article To Facilitate Evaluation of Objects Using Electromagnetic
Energy by the United States Office of Patents and Trademarks. The patent expires July 31, 2027.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company is pursuing an aggressive patent
strategy to expand our unique intellectual property in the United States and Japan and other countries.&lt;/p&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="From2011-10-01to2012-09-30" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="margin: 0pt"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company has developed a unique patented
Visualant Spectral Pattern Matching&amp;#153; technology. This technology directs structured light onto a physical substance to capture
a Visualant Spectral Signature &amp;#153; called a ChromaID&amp;#153;. When matched against existing databases, the ChromaID can be used
to identify, detect, or diagnose markers invisible to the human eye. ChromaID scanners can be integrated into a variety of mobile
or fixed-mount form factors, making it possible to effectively conduct analyses in the field that could only previously be performed
by large and expensive lab-based tests.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;Visualant Spectral Pattern Matching and
the ChromaID profile were developed over a seven year period by Professors Dr. Tom Furness and Dr. Brian Showngerdt of RATLab LLC
under contract to Visualant. The technology is now being transferred into products and a ScanHead module. Visualant has partnered
with Sumitomo Precision Products to manufacture the ScanHead and reduce the technology to a reliable and cost effective form. The
first demonstration of this is the Cyclops6 ChromaID Scanner which was demonstrated at the Japanese Instrumentation Manufacturing
Association trade show in Tokyo in October 2012. The Cyclops 6 ChromaID Scanner can be used to evaluate the technology for flat
surface applications and has sensitivity from 350nm to 1450nm.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;Visualant is pursuing an aggressive patent strategy to expand
our unique intellectual property in the United States and Japan. The following patents have been issued to date:&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On September 6, 2011, the Company announced that it was issued
US Patent No. 7,996,173, entitled &amp;#147;Method, Apparatus and Article to Facilitate Distributed Evaluation of Objects Using Electromagnetic
Energy,&amp;#148; by the United States Office of Patents and Trademarks.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On January 19, 2012, the Company announced
that it was issued US Patent No. 8,081,304, entitled &amp;#147;Method, Apparatus and Article to Facilitate Evaluation of Objects Using
Electromagnetic Energy&amp;#148; by the United States Office of Patents and Trademarks.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On March 20, 2012, the Company announced
that it was issued US Patent No. 8,076,630, entitled &amp;#147;System and Method of Evaluating an Object Using Electromagnetic Energy&amp;#148;
by the United States Office of Patents and Trademarks.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On November 1, 2012, the Company announced
that it was issued US Patent No. 8,285,510 entitled &amp;#147;System and Method of Evaluating an Object Using Electromagnetic Energy&amp;#148;
by the United States Office of Patents and Trademarks.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On June 7, 2011, the Company closed the
acquisition of all Visualant related assets of the RATLab LLC (&amp;#147;RATLab&amp;#148;). The RATLab is a Seattle based research and
development laboratory created by Dr. Tom Furness, founder and Director of the HITLab&amp;#160;International, with labs at Seattle,
University of Canterbury in New Zealand, and the University of Tasmania in Australia. With this acquisition, the Company consolidated
all intellectual property relating to the Spectral Pattern Matching technology except for environmental field of use. &amp;#160;The
Company acquired the Visualant related assets of the RATLab for (i) 1,000,000 shares of our common stock at closing valued at $0.20
per share, the price during the negotiation of this agreement; (ii) $250,000 (paid); and (iii) &amp;#160;payment of the outstanding
promissory note owing to Mr. Furness in the amount of $65,000 with accrued interest of $24,675 (paid).&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On July 31, 2012, the Company closed
the acquisition of the environmental field of use of its Spectral Pattern Matching technology from Javelin LLC (&amp;#147;Javelin&amp;#148;).
The Company acquired the Visualant related assets of Javelin for (i)&amp;#160;1,250,000 shares of our common stock at closing valued
at thirteen ($0.13) per share, the price during the negotiation of this agreement; and (ii)&amp;#160; $100,000, with $20,000 payable
at closing and $80,000 to be paid in four equal installments over a period of eight months. In addition, Company entered into a
Business Development Agreement which will pay Javelin ten percent (10%) on the gross margin received by Visualant from license
agreements and joint venture developments sourced by Javelin.&lt;/p&gt;



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