Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 28, 2012	Sep. 30, 2011	Sep. 28, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Revenues	$ 73,298	$ 65,951	$ 219,707	$ 204,530
Operating income	14,728	16,524	43,905	42,267
Capital expenditures	1,483	971	3,425	2,638
Depreciation and amortization	1,133	1,125	3,459	3,257
Engineering and other scientific
Segment Reporting Information [Line Items]
Revenues	53,071	47,785	158,784	151,233
Operating income	16,217	14,708	47,573	45,212
Capital expenditures	1,103	723	2,039	1,684
Depreciation and amortization	677	686	2,061	1,986
Environmental and health
Segment Reporting Information [Line Items]
Revenues	20,227	18,166	60,923	53,297
Operating income	6,933	5,691	20,722	15,794
Capital expenditures	174	86	348	287
Depreciation and amortization	72	64	209	168
Operating segments
Segment Reporting Information [Line Items]
Operating income	23,150	20,399	68,295	61,006
Capital expenditures	1,277	809	2,387	1,971
Depreciation and amortization	749	750	2,270	2,154
Corporate
Segment Reporting Information [Line Items]
Operating income	(8,422)	(3,875)	(24,390)	(18,739)
Capital expenditures	206	162	1,038	667
Depreciation and amortization	$ 384	$ 375	$ 1,189	$ 1,103